feat: show Marmot group relays as clickable icons with activity status

Replaces the comma-separated relay text on the Marmot Group Info
screen with a FlowRow of 55dp relay avatars that open RelayInfo on
tap and copy the URL on long-press, matching the relay-icon pattern
used elsewhere in the app.

To surface whether each configured relay is actually carrying traffic
for the group, MarmotGroupChatroom now tracks the most recent
kind:445 event timestamp observed from each delivering relay and the
info screen renders a green dot when a relay has delivered a group
event within the last 7 days (gray otherwise).

commons/src/commonMain/kotlin/com/vitorpamplona/amethyst/commons/model/marmotGroups/MarmotGroupChatroom.kt

@@ -27,9 +27,11 @@ import com.vitorpamplona.amethyst.commons.model.ListChange
 import com.vitorpamplona.amethyst.commons.model.Note
 import com.vitorpamplona.amethyst.commons.model.NotesGatherer
 import com.vitorpamplona.quartz.nip01Core.core.HexKey
+import com.vitorpamplona.quartz.nip01Core.relay.normalizer.NormalizedRelayUrl
 import kotlinx.coroutines.channels.BufferOverflow
 import kotlinx.coroutines.flow.MutableSharedFlow
 import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.update
 import java.lang.ref.WeakReference
 
 /**
@@ -51,6 +53,14 @@ class MarmotGroupChatroom(
     var newestMessage: Note? = null
     val unreadCount = MutableStateFlow(0)
 
+    /**
+     * Tracks the most recent createdAt (seconds) of a kind:445 group event
+     * observed from each relay, keyed by the relay that delivered it. Used by
+     * the Group Info screen to flag which configured relays are actively
+     * carrying traffic for this MLS group.
+     */
+    val relayActivity = MutableStateFlow<Map<NormalizedRelayUrl, Long>>(emptyMap())
+
     /**
      * True if the local user has ever sent an application message in this
      * group, OR explicitly created/owns it. Used by list UIs to split groups
@@ -134,6 +144,16 @@ class MarmotGroupChatroom(
         unreadCount.value = 0
     }
 
+    fun recordRelayActivity(
+        relay: NormalizedRelayUrl,
+        createdAt: Long,
+    ) {
+        relayActivity.update { current ->
+            val existing = current[relay] ?: 0L
+            if (createdAt > existing) current + (relay to createdAt) else current
+        }
+    }
+
     fun pruneMessagesToTheLatestOnly(): Set<Note> {
         val sorted = messages.sortedWith(DefaultFeedOrder)
         val toKeep =