feat: add Marmot group info screen, member list, and leave group

Based on MIP spec review, add missing UI screens:

MarmotManager additions (commons):
- memberPubkeys(): extract Nostr pubkeys from MLS ratchet tree
  leaf nodes via Credential.Basic identity
- memberCount(): get group member count
- groupEpoch(): expose current MLS epoch
- GroupMemberInfo data class for leaf index + pubkey pairs

Account & AccountViewModel:
- leaveMarmotGroup(): publish SelfRemove proposal and clean up
- marmotGroupMembers(): expose member list to UI

MarmotGroupInfoScreen (new):
- Group metadata header (name, ID, epoch, member count)
- Member list with leaf indices, clickable to view profiles
- "Leave Group" with confirmation dialog
- "Add Member" access from group info

Navigation:
- Route.MarmotGroupInfo added and wired into nav graph
- Chat screen title now clickable to navigate to group info

https://claude.ai/code/session_0194SxKfAU61PY92eqP4cCXM

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

@@ -35,6 +35,7 @@ import com.vitorpamplona.quartz.marmot.mip02Welcome.WelcomeEvent
 import com.vitorpamplona.quartz.marmot.mip03GroupMessages.GroupEvent
 import com.vitorpamplona.quartz.marmot.mls.group.MlsGroupManager
 import com.vitorpamplona.quartz.marmot.mls.group.MlsGroupStateStore
+import com.vitorpamplona.quartz.marmot.mls.tree.Credential
 import com.vitorpamplona.quartz.nip01Core.core.Event
 import com.vitorpamplona.quartz.nip01Core.core.HexKey
 import com.vitorpamplona.quartz.nip01Core.core.hexToByteArray
@@ -254,4 +255,44 @@ class MarmotManager(
      * Get all active group IDs.
      */
     fun activeGroupIds(): Set<HexKey> = groupManager.activeGroupIds()
+
+    // --- Group Info ---
+
+    /**
+     * Get the member count for a group.
+     */
+    fun memberCount(nostrGroupId: HexKey): Int = groupManager.getGroup(nostrGroupId)?.memberCount ?: 0
+
+    /**
+     * Get the member list for a group.
+     * Returns leaf index and Nostr pubkey (extracted from BasicCredential) for each member.
+     */
+    fun memberPubkeys(nostrGroupId: HexKey): List<GroupMemberInfo> {
+        val group = groupManager.getGroup(nostrGroupId) ?: return emptyList()
+        return group.members().mapNotNull { (leafIndex, leafNode) ->
+            val pubkey =
+                when (val cred = leafNode.credential) {
+                    is Credential.Basic -> cred.identity.toHexKey()
+                    else -> null
+                }
+            if (pubkey != null) {
+                GroupMemberInfo(leafIndex = leafIndex, pubkey = pubkey)
+            } else {
+                null
+            }
+        }
+    }
+
+    /**
+     * Get the current epoch for a group.
+     */
+    fun groupEpoch(nostrGroupId: HexKey): Long? = groupManager.getGroup(nostrGroupId)?.epoch
 }
+
+/**
+ * Information about a member in a Marmot MLS group.
+ */
+data class GroupMemberInfo(
+    val leafIndex: Int,
+    val pubkey: HexKey,
+)