feat: add blockchain explorer settings page for OTS verification

Adds a new "Bitcoin Explorer (OTS)" settings page in App Settings,
modeled after the existing Namecoin settings page.

Users can configure a custom Mempool-compatible blockchain explorer
API URL for OpenTimestamps proof verification. When a custom URL is
set it takes priority over the automatic Tor-aware selection (mempool.space
when Tor is active, blockstream.info otherwise).

Changes:
- OtsSettings: immutable data class for explorer config
- OtsSharedPreferences: DataStore-backed persistence (global/app-level)
- TorAwareOkHttpOtsResolverBuilder: accepts customExplorerUrl lambda
- OtsSettingsSection: UI section with active explorer display, URL input,
  known-explorers reference list, and reset button
- OtsSettingsScreen: full screen wrapping the section
- Routes.OtsSettings: new navigation route
- AllSettingsScreen: new entry under App Settings
- AppModules: wires otsPrefs and passes custom URL to resolver builder

https://claude.ai/code/session_01GX7k36uepGxU3U1fBKYFrx

amethyst/src/main/java/com/vitorpamplona/amethyst/AppModules.kt

@@ -29,6 +29,7 @@ import com.vitorpamplona.amethyst.model.LocalCache
 import com.vitorpamplona.amethyst.model.accountsCache.AccountCacheState
 import com.vitorpamplona.amethyst.model.nip03Timestamp.IncomingOtsEventVerifier
 import com.vitorpamplona.amethyst.model.nip03Timestamp.TorAwareOkHttpOtsResolverBuilder
+import com.vitorpamplona.amethyst.model.preferences.OtsSharedPreferences
 import com.vitorpamplona.amethyst.model.nip11RelayInfo.Nip11CachedRetriever
 import com.vitorpamplona.amethyst.model.preferences.NamecoinSharedPreferences
 import com.vitorpamplona.amethyst.model.preferences.TorSharedPreferences
@@ -111,6 +112,11 @@ class AppModules(
         NamecoinSharedPreferences(appContext, applicationIOScope)
     }
 
+    // OTS blockchain explorer preferences (global, like Tor settings)
+    val otsPrefs by lazy {
+        OtsSharedPreferences(appContext, applicationIOScope)
+    }
+
     // App services that should be run as soon as there are subscribers to their flows
     val locationManager = LocationState(appContext, applicationIOScope)
     val connManager = ConnectivityManager(appContext, applicationIOScope)
@@ -179,6 +185,7 @@ class AppModules(
             roleBasedHttpClientBuilder::okHttpClientForMoney,
             roleBasedHttpClientBuilder::shouldUseTorForMoneyOperations,
             otsBlockHeightCache,
+            customExplorerUrl = { otsPrefs.current.normalizedUrl() },
         )
 
     // Application-wide ots verification cache

amethyst/src/main/java/com/vitorpamplona/amethyst/model/nip03Timestamp/OtsSettings.kt

@@ -0,0 +1,70 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.amethyst.model.nip03Timestamp
+
+import androidx.compose.runtime.Stable
+import com.vitorpamplona.quartz.nip03Timestamp.okhttp.OkHttpBitcoinExplorer
+import kotlinx.serialization.Serializable
+
+/**
+ * Immutable data class representing the current OTS blockchain explorer config.
+ *
+ * When a custom URL is configured, it is used instead of the automatic
+ * Tor-aware selection (Mempool when Tor is active, Blockstream otherwise).
+ * This gives users control over which explorer observes their OTS verifications.
+ */
+@Serializable
+@Stable
+data class OtsSettings(
+    /**
+     * Custom blockchain explorer base API URL.
+     * When null/blank, the default Tor-aware selection is used.
+     * Must be a Mempool-compatible REST API (e.g. https://mempool.space/api/).
+     */
+    val customExplorerUrl: String? = null,
+) {
+    /** True when the user has configured a custom explorer URL. */
+    val hasCustomExplorer: Boolean get() = !customExplorerUrl.isNullOrBlank()
+
+    /**
+     * Returns the normalized custom URL (trailing slash ensured) or null if not set.
+     */
+    fun normalizedUrl(): String? {
+        val url = customExplorerUrl?.trim()?.takeIf { it.isNotBlank() } ?: return null
+        return if (url.endsWith("/")) url else "$url/"
+    }
+
+    companion object {
+        val DEFAULT = OtsSettings()
+
+        val KNOWN_EXPLORERS =
+            listOf(
+                OkHttpBitcoinExplorer.MEMPOOL_API_URL to "mempool.space (Tor-friendly)",
+                OkHttpBitcoinExplorer.BLOCKSTREAM_API_URL to "blockstream.info",
+            )
+
+        fun isValidUrl(url: String): Boolean {
+            val trimmed = url.trim()
+            if (trimmed.isBlank()) return false
+            return trimmed.startsWith("http://") || trimmed.startsWith("https://")
+        }
+    }
+}

amethyst/src/main/java/com/vitorpamplona/amethyst/model/nip03Timestamp/TorAwareOkHttpOtsResolverBuilder.kt

@@ -31,13 +31,15 @@ class TorAwareOkHttpOtsResolverBuilder(
     val okHttpClient: (url: String) -> OkHttpClient,
     val isTorActive: (url: String) -> Boolean,
     val cache: OtsBlockHeightCache,
+    val customExplorerUrl: () -> String? = { null },
 ) : OtsResolverBuilder {
-    fun getAPI(usingTor: Boolean) =
+    fun getAPI(usingTor: Boolean): String =
-        if (usingTor) {
+        customExplorerUrl()
-            OkHttpBitcoinExplorer.MEMPOOL_API_URL
+            ?: if (usingTor) {
-        } else {
+                OkHttpBitcoinExplorer.MEMPOOL_API_URL
-            OkHttpBitcoinExplorer.BLOCKSTREAM_API_URL
+            } else {
-        }
+                OkHttpBitcoinExplorer.BLOCKSTREAM_API_URL
+            }
 
     override fun build(): OtsResolver =
         OtsResolver(

amethyst/src/main/java/com/vitorpamplona/amethyst/model/preferences/OtsSharedPreferences.kt

@@ -0,0 +1,103 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.amethyst.model.preferences
+
+import android.content.Context
+import androidx.compose.runtime.Stable
+import androidx.datastore.preferences.core.edit
+import androidx.datastore.preferences.core.stringPreferencesKey
+import com.vitorpamplona.amethyst.model.nip03Timestamp.OtsSettings
+import com.vitorpamplona.quartz.utils.Log
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.flow.first
+import kotlinx.coroutines.runBlocking
+import kotlin.coroutines.cancellation.CancellationException
+
+/**
+ * Persistent storage for [OtsSettings], following the same pattern as
+ * [NamecoinSharedPreferences].
+ *
+ * Uses the app-wide [sharedPreferencesDataStore] so OTS explorer settings
+ * are global — not per-account.
+ */
+@Stable
+class OtsSharedPreferences(
+    private val context: Context,
+    private val scope: CoroutineScope,
+) {
+    companion object {
+        val KEY_CUSTOM_EXPLORER_URL = stringPreferencesKey("ots.customExplorerUrl")
+    }
+
+    /**
+     * Current settings, loaded synchronously at init to avoid races.
+     */
+    private val _settings =
+        MutableStateFlow(
+            runBlocking { loadFromDisk() ?: OtsSettings.DEFAULT },
+        )
+    val settings: StateFlow<OtsSettings> = _settings
+
+    /** Synchronous snapshot — safe to call from resolver builder lambdas. */
+    val current: OtsSettings get() = _settings.value
+
+    // ── Mutators ───────────────────────────────────────────────────────
+
+    suspend fun setCustomExplorerUrl(url: String?) {
+        val normalized = url?.trim()?.takeIf { it.isNotBlank() }
+        persist(current.copy(customExplorerUrl = normalized))
+    }
+
+    suspend fun reset() {
+        persist(OtsSettings.DEFAULT)
+    }
+
+    // ── Internal ───────────────────────────────────────────────────────
+
+    private suspend fun persist(settings: OtsSettings) {
+        _settings.value = settings
+        try {
+            context.sharedPreferencesDataStore.edit { prefs ->
+                if (settings.customExplorerUrl != null) {
+                    prefs[KEY_CUSTOM_EXPLORER_URL] = settings.customExplorerUrl
+                } else {
+                    prefs.remove(KEY_CUSTOM_EXPLORER_URL)
+                }
+            }
+        } catch (e: Exception) {
+            if (e is CancellationException) throw e
+            Log.e("OtsPrefs", "Error writing DataStore: ${e.message}")
+        }
+    }
+
+    private suspend fun loadFromDisk(): OtsSettings? =
+        try {
+            val prefs = context.sharedPreferencesDataStore.data.first()
+            val url = prefs[KEY_CUSTOM_EXPLORER_URL]?.takeIf { it.isNotBlank() }
+            OtsSettings(customExplorerUrl = url)
+        } catch (e: Exception) {
+            if (e is CancellationException) throw e
+            Log.e("OtsPrefs", "Error reading DataStore: ${e.message}")
+            null
+        }
+}

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/navigation/AppNavigation.kt

@@ -112,6 +112,7 @@ import com.vitorpamplona.amethyst.ui.screen.loggedIn.search.SearchScreen
 import com.vitorpamplona.amethyst.ui.screen.loggedIn.settings.AllSettingsScreen
 import com.vitorpamplona.amethyst.ui.screen.loggedIn.settings.NIP47SetupScreen
 import com.vitorpamplona.amethyst.ui.screen.loggedIn.settings.NamecoinSettingsScreen
+import com.vitorpamplona.amethyst.ui.screen.loggedIn.settings.OtsSettingsScreen
 import com.vitorpamplona.amethyst.ui.screen.loggedIn.settings.ReactionsSettingsScreen
 import com.vitorpamplona.amethyst.ui.screen.loggedIn.settings.SecurityFiltersScreen
 import com.vitorpamplona.amethyst.ui.screen.loggedIn.settings.SettingsScreen
@@ -177,6 +178,7 @@ fun AppNavigation(
             composableFromEnd<Route.SecurityFilters> { SecurityFiltersScreen(accountViewModel, nav) }
             composableFromEnd<Route.PrivacyOptions> { PrivacyOptionsScreen(Amethyst.instance.torPrefs.value, nav) }
             composableFromEnd<Route.NamecoinSettings> { NamecoinSettingsScreen(Amethyst.instance.namecoinPrefs, nav) }
+            composableFromEnd<Route.OtsSettings> { OtsSettingsScreen(Amethyst.instance.otsPrefs, Amethyst.instance.torPrefs.value, nav) }
             composableFromEnd<Route.Bookmarks> { BookmarkListScreen(accountViewModel, nav) }
             composableFromEnd<Route.Drafts> { DraftListScreen(accountViewModel, nav) }
             composableFromEnd<Route.Settings> { SettingsScreen(accountViewModel, nav) }

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/navigation/routes/Routes.kt

@@ -51,6 +51,8 @@ sealed class Route {
 
     @Serializable object NamecoinSettings : Route()
 
+    @Serializable object OtsSettings : Route()
+
     @Serializable object Bookmarks : Route()
 
     @Serializable object BookmarkGroups : Route()

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/settings/AllSettingsScreen.kt

@@ -31,6 +31,7 @@ import androidx.compose.material.icons.outlined.Bolt
 import androidx.compose.material.icons.outlined.CloudUpload
 import androidx.compose.material.icons.outlined.FavoriteBorder
 import androidx.compose.material.icons.outlined.Key
+import androidx.compose.material.icons.outlined.Search
 import androidx.compose.material.icons.outlined.Security
 import androidx.compose.material.icons.outlined.Settings
 import androidx.compose.material.icons.outlined.ThumbUp
@@ -153,6 +154,13 @@ fun AllSettingsScreen(
                 onClick = { nav.nav(Route.NamecoinSettings) },
             )
             HorizontalDivider()
+            SettingsNavigationRow(
+                title = R.string.ots_explorer_settings,
+                icon = Icons.Outlined.Search,
+                tint = tint,
+                onClick = { nav.nav(Route.OtsSettings) },
+            )
+            HorizontalDivider()
             SettingsNavigationRow(
                 title = R.string.ui_preferences,
                 icon = Icons.Outlined.Settings,

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/settings/OtsSettingsScreen.kt

@@ -0,0 +1,83 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.amethyst.ui.screen.loggedIn.settings
+
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.rememberScrollState
+import androidx.compose.foundation.verticalScroll
+import androidx.compose.material3.ExperimentalMaterial3Api
+import androidx.compose.material3.Scaffold
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.collectAsState
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.rememberCoroutineScope
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.unit.dp
+import com.vitorpamplona.amethyst.R
+import com.vitorpamplona.amethyst.model.preferences.OtsSharedPreferences
+import com.vitorpamplona.amethyst.ui.navigation.navs.INav
+import com.vitorpamplona.amethyst.ui.navigation.topbars.TopBarWithBackButton
+import com.vitorpamplona.amethyst.ui.stringRes
+import com.vitorpamplona.amethyst.ui.tor.TorSettingsFlow
+import com.vitorpamplona.amethyst.ui.tor.TorType
+import kotlinx.coroutines.launch
+
+@OptIn(ExperimentalMaterial3Api::class)
+@Composable
+fun OtsSettingsScreen(
+    otsPrefs: OtsSharedPreferences,
+    torSettings: TorSettingsFlow,
+    nav: INav,
+) {
+    val otsSettings by otsPrefs.settings.collectAsState()
+    val torType by torSettings.torType.collectAsState()
+    val moneyViaTor by torSettings.moneyOperationsViaTor.collectAsState()
+    val scope = rememberCoroutineScope()
+
+    val isTorActiveForMoney = torType != TorType.OFF && moneyViaTor
+
+    Scaffold(
+        topBar = {
+            TopBarWithBackButton(stringRes(id = R.string.ots_explorer_settings), nav::popBack)
+        },
+    ) {
+        Column(
+            Modifier
+                .padding(it)
+                .fillMaxSize()
+                .verticalScroll(rememberScrollState())
+                .padding(horizontal = 10.dp),
+        ) {
+            OtsSettingsSection(
+                settings = otsSettings,
+                isTorActive = isTorActiveForMoney,
+                onSetCustomUrl = { url ->
+                    scope.launch { otsPrefs.setCustomExplorerUrl(url) }
+                },
+                onReset = {
+                    scope.launch { otsPrefs.reset() }
+                },
+            )
+        }
+    }
+}

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/settings/OtsSettingsSection.kt

@@ -0,0 +1,369 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.amethyst.ui.screen.loggedIn.settings
+
+import androidx.compose.foundation.background
+import androidx.compose.foundation.border
+import androidx.compose.foundation.layout.Arrangement
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.Row
+import androidx.compose.foundation.layout.Spacer
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.height
+import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.layout.size
+import androidx.compose.foundation.layout.width
+import androidx.compose.foundation.shape.RoundedCornerShape
+import androidx.compose.foundation.text.KeyboardActions
+import androidx.compose.foundation.text.KeyboardOptions
+import androidx.compose.material.icons.Icons
+import androidx.compose.material.icons.filled.Clear
+import androidx.compose.material.icons.filled.Refresh
+import androidx.compose.material.icons.outlined.Search
+import androidx.compose.material3.HorizontalDivider
+import androidx.compose.material3.Icon
+import androidx.compose.material3.IconButton
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.OutlinedTextField
+import androidx.compose.material3.Text
+import androidx.compose.material3.TextButton
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.saveable.rememberSaveable
+import androidx.compose.runtime.setValue
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.graphics.Color
+import androidx.compose.ui.platform.LocalSoftwareKeyboardController
+import androidx.compose.ui.text.font.FontFamily
+import androidx.compose.ui.text.font.FontWeight
+import androidx.compose.ui.text.input.ImeAction
+import androidx.compose.ui.text.input.KeyboardType
+import androidx.compose.ui.unit.dp
+import com.vitorpamplona.amethyst.model.nip03Timestamp.OtsSettings
+import com.vitorpamplona.quartz.nip03Timestamp.okhttp.OkHttpBitcoinExplorer
+
+/**
+ * Settings section for configuring the blockchain explorer used for OTS verification.
+ *
+ * Displays the currently active explorer and allows the user to provide a
+ * custom Mempool-compatible REST API URL. When a custom URL is set, it takes
+ * priority over the automatic Tor-aware selection.
+ *
+ * @param settings         Current [OtsSettings] state
+ * @param isTorActive      Whether Tor is currently active (determines default explorer shown)
+ * @param onSetCustomUrl   Called with the trimmed URL string when user saves a custom URL
+ * @param onReset          Called when user clears the custom URL and reverts to auto-selection
+ */
+@Composable
+fun OtsSettingsSection(
+    settings: OtsSettings,
+    isTorActive: Boolean,
+    onSetCustomUrl: (String?) -> Unit,
+    onReset: () -> Unit,
+    modifier: Modifier = Modifier,
+) {
+    Column(modifier = modifier.padding(16.dp)) {
+        SectionHeaderOts()
+
+        Spacer(Modifier.height(12.dp))
+
+        Text(
+            "OpenTimestamps proofs are verified by querying a Bitcoin blockchain explorer. " +
+                "By default, mempool.space is used when Tor is active, and blockstream.info otherwise. " +
+                "Set a custom URL to use your own self-hosted instance or a trusted third party.",
+            style = MaterialTheme.typography.bodySmall,
+            color = MaterialTheme.colorScheme.onSurfaceVariant,
+        )
+
+        Spacer(Modifier.height(16.dp))
+
+        ActiveExplorerDisplay(settings = settings, isTorActive = isTorActive)
+
+        Spacer(Modifier.height(12.dp))
+        HorizontalDivider(
+            color = MaterialTheme.colorScheme.outlineVariant.copy(alpha = 0.5f),
+        )
+        Spacer(Modifier.height(12.dp))
+
+        CustomExplorerInput(
+            currentUrl = settings.customExplorerUrl,
+            onSave = onSetCustomUrl,
+        )
+
+        if (settings.hasCustomExplorer) {
+            Spacer(Modifier.height(4.dp))
+            Row(
+                modifier = Modifier.fillMaxWidth(),
+                horizontalArrangement = Arrangement.End,
+            ) {
+                TextButton(onClick = onReset) {
+                    Icon(
+                        Icons.Default.Refresh,
+                        contentDescription = null,
+                        modifier = Modifier.size(16.dp),
+                    )
+                    Spacer(Modifier.width(4.dp))
+                    Text("Reset to auto-select")
+                }
+            }
+        }
+
+        Spacer(Modifier.height(12.dp))
+        HorizontalDivider(
+            color = MaterialTheme.colorScheme.outlineVariant.copy(alpha = 0.5f),
+        )
+        Spacer(Modifier.height(12.dp))
+
+        KnownExplorersInfo()
+    }
+}
+
+// ── Sub-composables ────────────────────────────────────────────────────
+
+@Composable
+private fun SectionHeaderOts() {
+    Row(
+        modifier = Modifier.fillMaxWidth(),
+        verticalAlignment = Alignment.CenterVertically,
+    ) {
+        Icon(
+            Icons.Outlined.Search,
+            contentDescription = null,
+            tint = Color(0xFFF7931A), // Bitcoin orange
+            modifier = Modifier.size(22.dp),
+        )
+        Spacer(Modifier.width(10.dp))
+        Column {
+            Text(
+                "Blockchain Explorer",
+                style = MaterialTheme.typography.titleSmall,
+                fontWeight = FontWeight.SemiBold,
+            )
+            Text(
+                "Used for OTS timestamp verification",
+                style = MaterialTheme.typography.labelSmall,
+                color = MaterialTheme.colorScheme.onSurfaceVariant,
+            )
+        }
+    }
+}
+
+@Composable
+private fun ActiveExplorerDisplay(
+    settings: OtsSettings,
+    isTorActive: Boolean,
+) {
+    val isCustom = settings.hasCustomExplorer
+    val activeUrl =
+        settings.normalizedUrl()
+            ?: if (isTorActive) OkHttpBitcoinExplorer.MEMPOOL_API_URL else OkHttpBitcoinExplorer.BLOCKSTREAM_API_URL
+
+    Column {
+        Row(
+            modifier = Modifier.fillMaxWidth(),
+            horizontalArrangement = Arrangement.SpaceBetween,
+            verticalAlignment = Alignment.CenterVertically,
+        ) {
+            Text(
+                "Active explorer",
+                style = MaterialTheme.typography.labelMedium,
+                fontWeight = FontWeight.Medium,
+            )
+            if (isCustom) {
+                Text(
+                    "CUSTOM",
+                    style = MaterialTheme.typography.labelSmall,
+                    fontWeight = FontWeight.Bold,
+                    color = Color(0xFFF7931A),
+                    modifier =
+                        Modifier
+                            .background(
+                                Color(0xFFF7931A).copy(alpha = 0.1f),
+                                RoundedCornerShape(4.dp),
+                            ).padding(horizontal = 6.dp, vertical = 2.dp),
+                )
+            } else {
+                Text(
+                    if (isTorActive) "AUTO (TOR)" else "AUTO",
+                    style = MaterialTheme.typography.labelSmall,
+                    color = MaterialTheme.colorScheme.onSurfaceVariant,
+                )
+            }
+        }
+        Spacer(Modifier.height(6.dp))
+        Text(
+            text = activeUrl,
+            style = MaterialTheme.typography.bodySmall,
+            fontFamily = FontFamily.Monospace,
+            color = MaterialTheme.colorScheme.onSurface,
+            modifier =
+                Modifier
+                    .fillMaxWidth()
+                    .background(
+                        MaterialTheme.colorScheme.surfaceVariant.copy(alpha = 0.5f),
+                        RoundedCornerShape(6.dp),
+                    ).padding(horizontal = 10.dp, vertical = 6.dp),
+        )
+    }
+}
+
+@Composable
+private fun CustomExplorerInput(
+    currentUrl: String?,
+    onSave: (String?) -> Unit,
+) {
+    var input by rememberSaveable(currentUrl) { mutableStateOf(currentUrl ?: "") }
+    var validationError by remember { mutableStateOf<String?>(null) }
+    val kb = LocalSoftwareKeyboardController.current
+
+    fun trySave() {
+        val trimmed = input.trim()
+        if (trimmed.isBlank()) {
+            // Empty input means clear the custom URL
+            validationError = null
+            onSave(null)
+            kb?.hide()
+            return
+        }
+        if (!OtsSettings.isValidUrl(trimmed)) {
+            validationError = "Must start with http:// or https://"
+            return
+        }
+        validationError = null
+        onSave(trimmed)
+        kb?.hide()
+    }
+
+    Column {
+        Text(
+            "Custom explorer URL",
+            style = MaterialTheme.typography.labelMedium,
+            fontWeight = FontWeight.Medium,
+            modifier = Modifier.padding(bottom = 6.dp),
+        )
+        Row(
+            modifier = Modifier.fillMaxWidth(),
+            verticalAlignment = Alignment.Top,
+        ) {
+            OutlinedTextField(
+                value = input,
+                onValueChange = {
+                    input = it
+                    validationError = null
+                },
+                label = { Text("Explorer API base URL") },
+                placeholder = { Text("https://mempool.space/api/") },
+                singleLine = true,
+                isError = validationError != null,
+                supportingText =
+                    validationError?.let { err ->
+                        { Text(err, color = MaterialTheme.colorScheme.error) }
+                    },
+                trailingIcon =
+                    if (input.isNotBlank()) {
+                        {
+                            IconButton(onClick = {
+                                input = ""
+                                validationError = null
+                            }) {
+                                Icon(
+                                    Icons.Default.Clear,
+                                    contentDescription = "Clear",
+                                    modifier = Modifier.size(18.dp),
+                                )
+                            }
+                        }
+                    } else {
+                        null
+                    },
+                modifier = Modifier.weight(1f),
+                shape = RoundedCornerShape(8.dp),
+                keyboardOptions =
+                    KeyboardOptions(
+                        keyboardType = KeyboardType.Uri,
+                        imeAction = ImeAction.Done,
+                    ),
+                keyboardActions = KeyboardActions(onDone = { trySave() }),
+                textStyle =
+                    MaterialTheme.typography.bodySmall.copy(
+                        fontFamily = FontFamily.Monospace,
+                    ),
+            )
+            Spacer(Modifier.width(8.dp))
+            TextButton(
+                onClick = { trySave() },
+                modifier = Modifier.padding(top = 6.dp),
+            ) {
+                Text("Save")
+            }
+        }
+    }
+}
+
+@Composable
+private fun KnownExplorersInfo() {
+    Column {
+        Text(
+            "Known compatible explorers",
+            style = MaterialTheme.typography.labelMedium,
+            fontWeight = FontWeight.Medium,
+            modifier = Modifier.padding(bottom = 6.dp),
+        )
+        OtsSettings.KNOWN_EXPLORERS.forEach { (url, label) ->
+            Row(
+                modifier =
+                    Modifier
+                        .fillMaxWidth()
+                        .padding(vertical = 3.dp)
+                        .border(
+                            1.dp,
+                            MaterialTheme.colorScheme.outlineVariant.copy(alpha = 0.4f),
+                            RoundedCornerShape(6.dp),
+                        ).padding(horizontal = 10.dp, vertical = 6.dp),
+                verticalAlignment = Alignment.CenterVertically,
+                horizontalArrangement = Arrangement.SpaceBetween,
+            ) {
+                Column(modifier = Modifier.weight(1f)) {
+                    Text(
+                        text = label,
+                        style = MaterialTheme.typography.bodySmall,
+                        fontWeight = FontWeight.Medium,
+                    )
+                    Text(
+                        text = url,
+                        style = MaterialTheme.typography.labelSmall,
+                        fontFamily = FontFamily.Monospace,
+                        color = MaterialTheme.colorScheme.onSurfaceVariant,
+                    )
+                }
+            }
+        }
+        Spacer(Modifier.height(4.dp))
+        Text(
+            "Any Mempool-compatible REST API is supported (e.g. self-hosted mempool.space).",
+            style = MaterialTheme.typography.labelSmall,
+            color = MaterialTheme.colorScheme.onSurfaceVariant,
+        )
+    }
+}

amethyst/src/main/res/values/strings.xml

@@ -1748,4 +1748,5 @@
     <string name="select_all">Select All</string>
     <string name="uptime">%1$d%% uptime</string>
     <string name="namecoin_settings">Namecoin Settings</string>
+    <string name="ots_explorer_settings">Bitcoin Explorer (OTS)</string>
 </resources>