feat(TimeAgo): single shared ticker so on-screen ages stay fresh

`TimeAgo`, `NormalTimeAgo`, `ChatTimeAgo`, and the chatroom header's
private `TimeAgo` previously formatted the timestamp once inside
`remember(time)` and never refreshed — a note shown when it was 59 s
old would keep saying "59s" forever, even when it had been minutes.

Introduce a single app-wide ticker:

* `LocalNowSeconds` is a `CompositionLocal<State<Long>>` whose value
  is refreshed every 30 s by a single `produceState` coroutine inside
  `NowProvider` (mounted once at the app root in `MainActivity`).
* Each `TimeAgo` composable reads the ticker inside `derivedStateOf`,
  so it only triggers a Text recomposition when the formatted string
  actually crosses a threshold (e.g. 1m → 2m). Ticks that don't change
  the displayed string are filtered by `derivedStateOf` equality.

One coroutine total, no per-item timers, and recompositions are
proportional to "strings that actually need to update" rather than
"items on screen × ticks/second".

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/MainActivity.kt

@@ -37,6 +37,7 @@ import com.vitorpamplona.amethyst.service.playback.pip.BackgroundMedia
 import com.vitorpamplona.amethyst.ui.navigation.findParameterValue
 import com.vitorpamplona.amethyst.ui.navigation.routes.Route
 import com.vitorpamplona.amethyst.ui.navigation.routes.routeFor
+import com.vitorpamplona.amethyst.ui.note.elements.NowProvider
 import com.vitorpamplona.amethyst.ui.screen.AccountScreen
 import com.vitorpamplona.amethyst.ui.theme.AmethystTheme
 import com.vitorpamplona.quartz.nip01Core.core.AddressableEvent
@@ -76,7 +77,9 @@ class MainActivity : AppCompatActivity() {
         setContent {
             StringResSetup()
             AmethystTheme {
-                AccountScreen(Amethyst.instance.sessionManager)
+                NowProvider {
+                    AccountScreen(Amethyst.instance.sessionManager)
+                }
             }
         }
     }

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/note/elements/NowProvider.kt

@@ -0,0 +1,50 @@
+/*
+ * 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.note.elements
+
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.CompositionLocalProvider
+import androidx.compose.runtime.State
+import androidx.compose.runtime.compositionLocalOf
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.produceState
+import com.vitorpamplona.quartz.utils.TimeUtils
+import kotlinx.coroutines.delay
+
+private const val TICK_INTERVAL_MS = 30_000L
+
+// Shared coarse-grained "now" ticker. One coroutine refreshes the value at TICK_INTERVAL_MS,
+// every TimeAgo on screen reads from it. Because TimeAgo wraps the formatted string in
+// `derivedStateOf`, the Text only recomposes when the displayed string actually changes
+// (e.g. crossing 1m → 2m) — not on every tick.
+val LocalNowSeconds = compositionLocalOf<State<Long>> { mutableStateOf(TimeUtils.now()) }
+
+@Composable
+fun NowProvider(content: @Composable () -> Unit) {
+    val now =
+        produceState(TimeUtils.now()) {
+            while (true) {
+                delay(TICK_INTERVAL_MS)
+                value = TimeUtils.now()
+            }
+        }
+    CompositionLocalProvider(LocalNowSeconds provides now, content = content)
+}

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/note/elements/TimeAgo.kt

@@ -25,7 +25,6 @@ import androidx.compose.material3.Text
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.derivedStateOf
 import androidx.compose.runtime.getValue
-import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.platform.LocalContext
@@ -46,7 +45,16 @@ fun TimeAgo(note: Note) {
 @Composable
 fun TimeAgo(time: Long) {
     val context = LocalContext.current
-    val timeStr by remember(time) { mutableStateOf(timeAgo(time, context = context)) }
+    // Subscribe to the shared coarse ticker; `derivedStateOf` ensures the Text only
+    // recomposes when the formatted string actually flips (e.g. 1m → 2m), not on every tick.
+    val nowState = LocalNowSeconds.current
+    val timeStr by
+        remember(time, context, nowState) {
+            derivedStateOf {
+                nowState.value
+                timeAgo(time, context = context)
+            }
+        }
 
     Text(
         text = timeStr,
@@ -61,9 +69,15 @@ fun NormalTimeAgo(
     modifier: Modifier,
 ) {
     val nowStr = stringRes(id = R.string.now)
+    val nowState = LocalNowSeconds.current
 
     val time by
-        remember(baseNote) { derivedStateOf { timeAgoShort(baseNote.createdAt() ?: 0L, nowStr) } }
+        remember(baseNote, nowStr, nowState) {
+            derivedStateOf {
+                nowState.value
+                timeAgoShort(baseNote.createdAt() ?: 0L, nowStr)
+            }
+        }
 
     Text(
         text = time,

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/chats/feed/ChatTimeAgo.kt

@@ -26,6 +26,8 @@ import androidx.compose.foundation.layout.size
 import androidx.compose.material3.MaterialTheme
 import androidx.compose.material3.Text
 import androidx.compose.runtime.Composable
+import androidx.compose.runtime.derivedStateOf
+import androidx.compose.runtime.getValue
 import androidx.compose.runtime.remember
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
@@ -35,6 +37,7 @@ import com.vitorpamplona.amethyst.R
 import com.vitorpamplona.amethyst.commons.icons.symbols.Icon
 import com.vitorpamplona.amethyst.commons.icons.symbols.MaterialSymbols
 import com.vitorpamplona.amethyst.model.Note
+import com.vitorpamplona.amethyst.ui.note.elements.LocalNowSeconds
 import com.vitorpamplona.amethyst.ui.note.timeAgoShort
 import com.vitorpamplona.amethyst.ui.note.timeAheadNoDot
 import com.vitorpamplona.amethyst.ui.stringRes
@@ -46,7 +49,14 @@ import com.vitorpamplona.quartz.nip40Expiration.expiration
 @Composable
 fun ChatTimeAgo(baseNote: Note) {
     val nowStr = stringRes(id = R.string.now)
-    val time = remember(baseNote) { timeAgoShort(baseNote.createdAt() ?: 0L, nowStr) }
+    val nowState = LocalNowSeconds.current
+    val time by
+        remember(baseNote, nowStr, nowState) {
+            derivedStateOf {
+                nowState.value
+                timeAgoShort(baseNote.createdAt() ?: 0L, nowStr)
+            }
+        }
 
     Text(
         text = time,

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/chats/rooms/ChatroomHeaderCompose.kt

@@ -27,6 +27,7 @@ import androidx.compose.material3.MaterialTheme
 import androidx.compose.material3.Text
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.LaunchedEffect
+import androidx.compose.runtime.derivedStateOf
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
@@ -60,6 +61,7 @@ import com.vitorpamplona.amethyst.ui.note.LoadDecryptedContentOrNull
 import com.vitorpamplona.amethyst.ui.note.LoadPublicChatChannel
 import com.vitorpamplona.amethyst.ui.note.NonClickableUserPictures
 import com.vitorpamplona.amethyst.ui.note.ObserveDraftEvent
+import com.vitorpamplona.amethyst.ui.note.elements.LocalNowSeconds
 import com.vitorpamplona.amethyst.ui.note.timeAgo
 import com.vitorpamplona.amethyst.ui.screen.loggedIn.AccountViewModel
 import com.vitorpamplona.amethyst.ui.screen.loggedIn.chats.privateDM.header.RoomNameDisplay
@@ -483,7 +485,14 @@ private fun TimeAgo(channelLastTime: Long?) {
     if (channelLastTime == null) return
 
     val context = LocalContext.current
-    val timeAgo = remember(channelLastTime) { timeAgo(channelLastTime, context) }
+    val nowState = LocalNowSeconds.current
+    val timeAgo by
+        remember(channelLastTime, context, nowState) {
+            derivedStateOf {
+                nowState.value
+                timeAgo(channelLastTime, context)
+            }
+        }
     Text(
         text = timeAgo,
         color = MaterialTheme.colorScheme.grayText,