feat: add ChatScrollController and JumpToBottomButton for improved chat scrolling experience

- Implemented ChatScrollController to manage scroll behavior and visibility of jump-to-bottom button.
- Added functionality to automatically scroll to the bottom when the keyboard opens.
- Created JumpToBottomButton widget that appears when the user scrolls up, allowing quick navigation back to the bottom of the chat.

lib/connector/meshcore_connector.dart

@@ -146,6 +146,7 @@ class MeshCoreConnector extends ChangeNotifier {
   final Set<String> _knownContactKeys = {};
   final Map<String, int> _contactLastReadMs = {};
   final Map<int, int> _channelLastReadMs = {};
+  bool _unreadStateLoaded = false;
   final Map<String, _RepeaterAckContext> _pendingRepeaterAcks = {};
   String? _activeContactKey;
   int? _activeChannelIndex;
@@ -317,6 +318,7 @@ class MeshCoreConnector extends ChangeNotifier {
   }
 
   int getUnreadCountForContactKey(String contactKeyHex) {
+    if (!_unreadStateLoaded) return 0;
     if (!_shouldTrackUnreadForContactKey(contactKeyHex)) return 0;
     final messages = _conversations[contactKeyHex];
     if (messages == null || messages.isEmpty) return 0;
@@ -336,6 +338,7 @@ class MeshCoreConnector extends ChangeNotifier {
   }
 
   int getUnreadCountForChannelIndex(int channelIndex) {
+    if (!_unreadStateLoaded) return 0;
     final messages = _channelMessages[channelIndex];
     if (messages == null || messages.isEmpty) return 0;
     final lastReadMs = _channelLastReadMs[channelIndex] ?? 0;
@@ -350,6 +353,7 @@ class MeshCoreConnector extends ChangeNotifier {
   }
 
   int getTotalUnreadCount() {
+    if (!_unreadStateLoaded) return 0;
     var total = 0;
     // Count unread contact messages
     for (final contact in _contacts) {
@@ -381,6 +385,7 @@ class MeshCoreConnector extends ChangeNotifier {
     _channelLastReadMs
       ..clear()
       ..addAll(await _unreadStore.loadChannelLastRead());
+    _unreadStateLoaded = true;
     notifyListeners();
   }