Phase 3.2 WS1: Chat request overlay, queue, stale reasons

- Backend: add reason field to chat_request_closed WS messages
  (cancelled_by_customer, accepted_by_other, expired)
- Backend: include duration_minutes, is_free_trial in chat_request WS
- ChatRequestNotifier: add ChatRequestStaleData, StaleReason enum,
  request queue (List<Map>), ignore(), acknowledgeStale(), _advanceQueue()
- New ChatRequestOverlay widget: slides up from bottom, dimmed background,
  swipe to dismiss, shows active/stale request content
- Integrate overlay in main.dart wrapping MaterialApp.router
- Cleanup: convert HomeScreen to ConsumerWidget, remove showModalBottomSheet,
  remove IncomingRequestSheet, remove lifecycle observer

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

mitra_app/lib/main.dart

@@ -6,6 +6,7 @@ import 'core/api/api_client_provider.dart';
 import 'core/auth/auth_notifier.dart';
 import 'core/status/status_notifier.dart';
 import 'core/chat/chat_request_notifier.dart';
+import 'core/chat/widgets/chat_request_overlay.dart';
 import 'core/notifications/notification_service.dart';
 import 'firebase_options.dart';
 import 'router.dart';
@@ -83,9 +84,11 @@ class _AppState extends ConsumerState<App> with WidgetsBindingObserver {
       ref.read(chatRequestProvider.notifier).setIncomingFromNotification(sessionId);
     };
 
-    return MaterialApp.router(
-      title: 'Halo Bestie Mitra',
-      routerConfig: router,
+    return ChatRequestOverlay(
+      child: MaterialApp.router(
+        title: 'Halo Bestie Mitra',
+        routerConfig: router,
+      ),
     );
   }
 }