fix: route Leave Group back to the Messages screen

After leaving a Marmot group the user was dropped on the group list;
send them to the main Messages tab instead, which is the correct
starting point for picking another conversation.

https://claude.ai/code/session_01JaeqZwVNLKvUUvXWRzPmRP

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/chats/marmotGroup/MarmotGroupInfoScreen.kt

@@ -301,7 +301,7 @@ fun MarmotGroupInfoScreen(
                     try {
                         accountViewModel.leaveMarmotGroup(nostrGroupId)
                         accountViewModel.account.marmotGroupList.removeGroup(nostrGroupId)
-                        nav.nav(Route.MarmotGroupList)
+                        nav.nav(Route.Message)
                     } catch (e: Exception) {
                         isLeaving = false
                         launch(Dispatchers.Main) {