feat: Improve offline detection logic

This commit refactors the connectivity service to be more robust and less prone to UI flicker.

Key changes:
- Any successful API response now briefly suppresses the offline warning. This prevents the UI from flashing an offline message between regular connectivity checks.
- The threshold for showing the offline warning is increased from 2 to 3 consecutive failed health checks.
- The timeout for health checks is increased to better handle slow networks.
- The offline warning is now suppressed if there are active data streams to avoid interrupting the user.
- A custom JSON converter is added for conversation metadata to handle potential type mismatches from local storage.

lib/core/router/app_router.dart

@@ -6,6 +6,7 @@ import 'package:go_router/go_router.dart';
 import '../providers/app_providers.dart';
 import '../services/connectivity_service.dart';
 import '../services/navigation_service.dart';
+import '../services/persistent_streaming_service.dart';
 import '../utils/debug_logger.dart';
 import '../../features/auth/providers/unified_auth_providers.dart';
 import '../../features/auth/views/authentication_page.dart';
@@ -103,12 +104,16 @@ class RouterNotifier extends ChangeNotifier {
     // 1. Not in reviewer mode
     // 2. Connectivity is explicitly offline
     // 3. Auth is authenticated (don't interrupt auth flow)
+    // 4. App is in foreground and offline warning isn't suppressed
+    // 5. No active streaming is in progress (avoid interrupting token streams)
+    final hasActiveStreams = PersistentStreamingService().activeStreamCount > 0;
     final shouldShowConnectionIssue =
         !reviewerMode &&
         connectivity == ConnectivityStatus.offline &&
         authState == AuthNavigationState.authenticated &&
         connectivityService.isAppForeground &&
-        !connectivityService.isOfflineSuppressed;
+        !connectivityService.isOfflineSuppressed &&
+        !hasActiveStreams;
 
     if (shouldShowConnectionIssue) {
       return location == Routes.connectionIssue ? null : Routes.connectionIssue;