feat: read MIP-01 group metadata from MLS GroupContext extensions

Implement end-to-end reading of MarmotGroupData (extension 0xF2EE)
from the MLS GroupContext.extensions and sync to the UI layer.

Quartz (protocol):
- MarmotGroupData.decodeTls(): deserialize from TLS wire format
  (version, nostrGroupId, name, description, admin pubkeys, relays,
  image fields)
- MarmotGroupData.fromExtensions(): find and decode from extension list
- MlsGroup.extensions: expose groupContext.extensions publicly

Commons (shared logic):
- MarmotGroupChatroom: add description, adminPubkeys, relays fields
  as MutableStateFlow for reactive UI
- MarmotManager.groupMetadata(): extract MIP-01 data from group
- MarmotManager.syncMetadataTo(): sync metadata + member count to
  a MarmotGroupChatroom

Sync points (amethyst):
- Account init: sync metadata after restoreAll() for all restored groups
- GiftWrapEventHandler: sync after Welcome processing (group join)
- GroupEventHandler: sync after CommitProcessed (epoch advances may
  update extensions via GroupContextExtensions proposals)

UI (GroupInfoScreen):
- Display group description from MIP-01 metadata
- Display relay URLs from MIP-01 metadata
- Mark admin members with "- admin" suffix (from adminPubkeys)

https://claude.ai/code/session_0194SxKfAU61PY92eqP4cCXM

commons/src/commonMain/kotlin/com/vitorpamplona/amethyst/commons/marmot/MarmotManager.kt

@@ -20,6 +20,7 @@
  */
 package com.vitorpamplona.amethyst.commons.marmot
 
+import com.vitorpamplona.amethyst.commons.model.marmotGroups.MarmotGroupChatroom
 import com.vitorpamplona.quartz.marmot.GroupEventResult
 import com.vitorpamplona.quartz.marmot.MarmotInboundProcessor
 import com.vitorpamplona.quartz.marmot.MarmotOutboundProcessor
@@ -31,6 +32,7 @@ import com.vitorpamplona.quartz.marmot.WelcomeResult
 import com.vitorpamplona.quartz.marmot.mip00KeyPackages.KeyPackageEvent
 import com.vitorpamplona.quartz.marmot.mip00KeyPackages.KeyPackageRotationManager
 import com.vitorpamplona.quartz.marmot.mip00KeyPackages.KeyPackageUtils
+import com.vitorpamplona.quartz.marmot.mip01Groups.MarmotGroupData
 import com.vitorpamplona.quartz.marmot.mip02Welcome.WelcomeEvent
 import com.vitorpamplona.quartz.marmot.mip03GroupMessages.GroupEvent
 import com.vitorpamplona.quartz.marmot.mls.group.MlsGroupManager
@@ -287,6 +289,37 @@ class MarmotManager(
      * Get the current epoch for a group.
      */
     fun groupEpoch(nostrGroupId: HexKey): Long? = groupManager.getGroup(nostrGroupId)?.epoch
+
+    /**
+     * Get the MIP-01 group metadata from the MLS GroupContext extensions.
+     * Returns null if the group doesn't exist or has no MarmotGroupData extension.
+     */
+    fun groupMetadata(nostrGroupId: HexKey): MarmotGroupData? {
+        val group = groupManager.getGroup(nostrGroupId) ?: return null
+        return MarmotGroupData.fromExtensions(group.extensions)
+    }
+
+    /**
+     * Sync MIP-01 metadata and member info from the MLS group into a [MarmotGroupChatroom].
+     * Call after joining a group, processing a commit, or restoring from storage.
+     */
+    fun syncMetadataTo(
+        nostrGroupId: HexKey,
+        chatroom: MarmotGroupChatroom,
+    ) {
+        val metadata = groupMetadata(nostrGroupId)
+        if (metadata != null) {
+            if (metadata.name.isNotEmpty()) {
+                chatroom.displayName.value = metadata.name
+            }
+            if (metadata.description.isNotEmpty()) {
+                chatroom.description.value = metadata.description
+            }
+            chatroom.adminPubkeys.value = metadata.adminPubkeys
+            chatroom.relays.value = metadata.relays
+        }
+        chatroom.memberCount.value = memberCount(nostrGroupId)
+    }
 }
 
 /**

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

@@ -42,6 +42,9 @@ class MarmotGroupChatroom(
 ) : NotesGatherer {
     var messages: Set<Note> = setOf()
     var displayName = MutableStateFlow<String?>(null)
+    var description = MutableStateFlow<String?>(null)
+    var adminPubkeys = MutableStateFlow<List<HexKey>>(emptyList())
+    var relays = MutableStateFlow<List<String>>(emptyList())
     var memberCount = MutableStateFlow(0)
     var newestMessage: Note? = null