feat(ui): relay health indicator in NavigationRail

Add RelayHealthIndicator component — shows elapsed time since last
relay event. Hidden when <30s (healthy), shows "45s ago" / "3m ago"
when stale. Placed above BunkerHeartbeatIndicator in NavigationRail.
Reads from Coordinator's subscriptionHealth StateFlow.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

desktopApp/src/jvmMain/kotlin/com/vitorpamplona/amethyst/desktop/Main.kt

@@ -736,6 +736,7 @@ fun MainContent(
             Row(Modifier.fillMaxSize().weight(1f)) {
                 when (layoutMode) {
                     LayoutMode.SINGLE_PANE -> {
+                        val healthMap by subscriptionsCoordinator.subscriptionHealth.collectAsState()
                         SinglePaneLayout(
                             relayManager = relayManager,
                             localCache = localCache,
@@ -752,6 +753,7 @@ fun MainContent(
                             onZapFeedback = onZapFeedback,
                             signerConnectionState = signerConnectionState,
                             lastPingTimeSec = lastPingTimeSec,
+                            subscriptionHealth = healthMap,
                             modifier = Modifier.weight(1f),
                         )
                     }

desktopApp/src/jvmMain/kotlin/com/vitorpamplona/amethyst/desktop/ui/components/RelayHealthIndicator.kt

@@ -0,0 +1,76 @@
+/*
+ * 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.desktop.ui.components
+
+import androidx.compose.foundation.layout.padding
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Text
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.LaunchedEffect
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableLongStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.setValue
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.unit.dp
+import kotlinx.coroutines.delay
+
+/**
+ * Displays relay health as elapsed time since last event.
+ * Hidden when < 30s (healthy). Shows "45s" or "3m" when stale.
+ */
+@Composable
+fun RelayHealthIndicator(
+    lastEventReceivedAt: Long?,
+    modifier: Modifier = Modifier,
+) {
+    if (lastEventReceivedAt == null) return
+
+    // Tick every 5s to update elapsed display
+    var now by remember { mutableLongStateOf(System.currentTimeMillis()) }
+    LaunchedEffect(Unit) {
+        while (true) {
+            delay(5_000)
+            now = System.currentTimeMillis()
+        }
+    }
+
+    val elapsedMs = now - lastEventReceivedAt
+    if (elapsedMs < 30_000) return // healthy, don't show
+
+    val text = formatElapsed(elapsedMs)
+
+    Text(
+        text = text,
+        style = MaterialTheme.typography.labelSmall,
+        color = MaterialTheme.colorScheme.onSurfaceVariant,
+        modifier = modifier.padding(horizontal = 8.dp),
+    )
+}
+
+private fun formatElapsed(elapsedMs: Long): String {
+    val seconds = elapsedMs / 1000
+    return when {
+        seconds < 60 -> "${seconds}s ago"
+        seconds < 3600 -> "${seconds / 60}m ago"
+        else -> "${seconds / 3600}h ago"
+    }
+}

desktopApp/src/jvmMain/kotlin/com/vitorpamplona/amethyst/desktop/ui/deck/SinglePaneLayout.kt

@@ -65,7 +65,9 @@ import com.vitorpamplona.amethyst.desktop.cache.DesktopLocalCache
 import com.vitorpamplona.amethyst.desktop.network.DesktopRelayConnectionManager
 import com.vitorpamplona.amethyst.desktop.service.highlights.DesktopHighlightStore
 import com.vitorpamplona.amethyst.desktop.subscriptions.DesktopRelaySubscriptionsCoordinator
+import com.vitorpamplona.amethyst.desktop.subscriptions.SubscriptionHealth
 import com.vitorpamplona.amethyst.desktop.ui.ZapFeedback
+import com.vitorpamplona.amethyst.desktop.ui.components.RelayHealthIndicator
 import com.vitorpamplona.amethyst.desktop.ui.media.LocalIsImmersiveFullscreen
 import com.vitorpamplona.quartz.nip47WalletConnect.Nip47WalletConnect.Nip47URINorm
 import kotlinx.coroutines.CoroutineScope
@@ -108,6 +110,7 @@ fun SinglePaneLayout(
     onZapFeedback: (ZapFeedback) -> Unit,
     signerConnectionState: SignerConnectionState,
     lastPingTimeSec: Long?,
+    subscriptionHealth: Map<String, SubscriptionHealth> = emptyMap(),
     modifier: Modifier = Modifier,
 ) {
     var currentColumnType by remember { mutableStateOf<DeckColumnType>(DeckColumnType.HomeFeed) }
@@ -150,6 +153,16 @@ fun SinglePaneLayout(
 
                 Spacer(Modifier.weight(1f))
 
+                // Relay health — shows elapsed time since last event (hidden when <30s)
+                val latestEvent =
+                    subscriptionHealth.values
+                        .mapNotNull { it.lastEventReceivedAt }
+                        .maxOrNull()
+                RelayHealthIndicator(
+                    lastEventReceivedAt = latestEvent,
+                    modifier = Modifier.padding(bottom = 4.dp),
+                )
+
                 BunkerHeartbeatIndicator(
                     signerConnectionState = signerConnectionState,
                     lastPingTimeSec = lastPingTimeSec,