fix(chat): fix jump-to-oldest-unread scroll not reaching target message

- Pass initialUnreadCount to chat screens before markRead clears it
- Use two-phase scroll: jumpTo estimated offset to build lazy items,
  then ensureVisible for precise positioning
- Await ensureVisible before clearing scroll guard to prevent
  scrollToBottomIfAtBottom from overriding the animation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/screens/map_screen.dart

@@ -1391,11 +1391,18 @@ class _MapScreenState extends State<MapScreen> {
         room: room,
         // onLogin(password, isAdmin) isAdmin not used for room caht screen
         onLogin: (password, _) {
-          // Navigate to chat screen after successful login
-          context.read<MeshCoreConnector>().markContactRead(room.publicKeyHex);
+          final connector = context.read<MeshCoreConnector>();
+          final unread =
+              connector.getUnreadCountForContactKey(room.publicKeyHex);
+          connector.markContactRead(room.publicKeyHex);
           Navigator.push(
             context,
-            MaterialPageRoute(builder: (context) => ChatScreen(contact: room)),
+            MaterialPageRoute(
+              builder: (context) => ChatScreen(
+                contact: room,
+                initialUnreadCount: unread,
+              ),
+            ),
           );
         },
       ),