fix: eliminate pull-to-refresh flicker by holding stale data during re-fetch

lib/providers/chat_provider.dart

@@ -58,6 +58,12 @@ class ConversationsNotifier extends AsyncNotifier<List<Conversation>> {
     ]);
   }
 
+  /// Re-fetch conversations without clearing the current list (no flicker).
+  Future<void> refresh() async {
+    final fresh = await ref.read(chatRepositoryProvider).getConversations();
+    state = AsyncData(fresh);
+  }
+
   // Called after a message is sent to patch the server-generated title
   // in-place without triggering a full reload or loading state.
   void patchConversation(Conversation updated) {
@@ -86,6 +92,13 @@ class MessagesNotifier extends AsyncNotifier<List<Message>> {
     return messages;
   }
 
+  /// Re-fetch messages without clearing the current list (no flicker).
+  Future<void> refresh() async {
+    final (_, messages) =
+        await ref.read(chatRepositoryProvider).getMessages(_convId);
+    state = AsyncData(messages);
+  }
+
   Future<void> sendMessage(String content) async {
     final convId = _convId;
     final repo = ref.read(chatRepositoryProvider);