refactor: centralize conversation cache management

- Introduced a new method `refreshConversationsCache` to streamline the invalidation of the conversations provider and optionally the folders provider.
- Updated various components to utilize the new cache management method, enhancing code clarity and reducing redundancy.
- This refactor improves the efficiency of conversation and folder synchronization across the application.

lib/core/providers/app_providers.dart

@@ -661,6 +661,17 @@ class _ConversationsCacheTimestamp extends _$ConversationsCacheTimestamp {
   void set(DateTime? timestamp) => state = timestamp;
 }
 
+/// Clears the in-memory timestamp cache and invalidates the conversations
+/// provider so the next read forces a refetch. Optionally invalidates the
+/// folders provider when folder metadata must stay in sync with conversations.
+void refreshConversationsCache(dynamic ref, {bool includeFolders = false}) {
+  ref.read(_conversationsCacheTimestampProvider.notifier).set(null);
+  ref.invalidate(conversationsProvider);
+  if (includeFolders) {
+    ref.invalidate(foldersProvider);
+  }
+}
+
 // Conversation providers - Now using correct OpenWebUI API with caching
 // keepAlive to maintain cache during authenticated session
 @Riverpod(keepAlive: true)

lib/core/providers/app_startup_providers.dart

@@ -103,7 +103,7 @@ void _scheduleConversationWarmup(Ref ref, {bool force = false}) {
         return;
       }
       if (existing.hasError) {
-        ref.invalidate(conversationsProvider);
+        refreshConversationsCache(ref);
       }
       final conversations = await ref.read(conversationsProvider.future);
       statusController.set(_ConversationWarmupStatus.complete);
@@ -368,7 +368,7 @@ class _ForegroundRefreshObserver extends WidgetsBindingObserver {
       // Schedule to avoid side-effects during build frames
       Future.microtask(() {
         try {
-          _ref.invalidate(conversationsProvider);
+          refreshConversationsCache(_ref);
           _ref
               .read(_conversationWarmupStatusProvider.notifier)
               .set(_ConversationWarmupStatus.idle);