fix(concord): round the create-community FAB to match app convention

The hub's create FAB fell back to Material3's default rounded-square shape;
every other FAB in the app (RelayGroupChannelListScreen, MarmotGroupListScreen,
NewNoteButton, …) uses shape = CircleShape with a 24dp icon. Match it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CzJ2Cwo8tg4oZq43oRa3ig

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/chats/publicChannels/concord/ConcordHomeScreen.kt

@@ -88,8 +88,12 @@ fun ConcordHomeScreen(
             )
         },
         floatingActionButton = {
-            FloatingActionButton(onClick = { nav.nav(Route.ConcordCreate) }) {
-                SymbolIcon(symbol = MaterialSymbols.Add, contentDescription = stringRes(R.string.concord_create_title))
+            FloatingActionButton(onClick = { nav.nav(Route.ConcordCreate) }, shape = CircleShape) {
+                SymbolIcon(
+                    symbol = MaterialSymbols.Add,
+                    contentDescription = stringRes(R.string.concord_create_title),
+                    modifier = Modifier.size(24.dp),
+                )
             }
         },
     ) { padding ->