Phase 3 testing fixes: Fastify 5, SSE→WebSocket+FCM, enums, security, session lifecycle

- Upgrade Fastify 4→5 with all plugins (@fastify/websocket 11, cors 11, sensible 6)
- Migrate all SSE endpoints to WebSocket + FCM push (mitra chat requests, customer pairing status)
- Add flutter_local_notifications for foreground push notifications with sound
- Add splash screen to both apps (hide auth loading flash)
- Introduce constants/enums across entire codebase (no raw string literals)
- Move price tiers from hardcoded array to app_config DB (data-driven, includes 1-min test tier)
- Add session ownership validation on all shared chat routes
- Add ownership checks on endSession, respondToExtension, requestExtension
- Fix session timer: auto-complete expired/stale sessions on server restart
- Add 5-min grace period for abandoned closing sessions
- Fix extension flow: proper session_resumed handling, clearExtensionRequest, closure grace timer cleanup
- Fix chat screens: ConnectChat in initState, session status check on connect
- Fix customer expired view: 5-min countdown, closure state priority over expired state
- Fix mitra extension UI: loading spinner, disable buttons, handle EXTENSION_RESOLVED error
- Fix GoRouter navigation consistency (no more Navigator.pushNamed)
- Fix goodbye view keyboard overflow (SingleChildScrollView)
- Add active session card on customer home screen with refresh on navigate back
- Fix PricingBottomSheet extension mode (RequestExtension instead of new pairing)
- Send session_resumed to both parties on extension accept

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

mitra_app/lib/core/notifications/notification_service.dart

@@ -0,0 +1,95 @@
+import 'dart:convert';
+import 'package:firebase_messaging/firebase_messaging.dart';
+import 'package:flutter_local_notifications/flutter_local_notifications.dart';
+import 'package:go_router/go_router.dart';
+
+/// Handles FCM foreground/background notifications and local notification display.
+class NotificationService {
+  static final _localNotifications = FlutterLocalNotificationsPlugin();
+  static GoRouter? _router;
+
+  static const _channel = AndroidNotificationChannel(
+    'chat_messages',
+    'Chat Messages',
+    description: 'Notifications for incoming chat messages',
+    importance: Importance.high,
+  );
+
+  static Future<void> initialize(GoRouter router) async {
+    _router = router;
+
+    // Create Android notification channel
+    await _localNotifications
+        .resolvePlatformSpecificImplementation<AndroidFlutterLocalNotificationsPlugin>()
+        ?.createNotificationChannel(_channel);
+
+    // Initialize local notifications
+    await _localNotifications.initialize(
+      settings: const InitializationSettings(
+        android: AndroidInitializationSettings('@mipmap/ic_launcher'),
+        iOS: DarwinInitializationSettings(),
+      ),
+      onDidReceiveNotificationResponse: _onNotificationTap,
+    );
+
+    // FCM foreground messages → show local notification
+    FirebaseMessaging.onMessage.listen(_onForegroundMessage);
+
+    // FCM notification tap (app was in background)
+    FirebaseMessaging.onMessageOpenedApp.listen(_onMessageOpenedApp);
+
+    // Check if app was opened from a terminated state via notification
+    final initialMessage = await FirebaseMessaging.instance.getInitialMessage();
+    if (initialMessage != null) {
+      _navigateFromMessage(initialMessage.data);
+    }
+  }
+
+  static void _onForegroundMessage(RemoteMessage message) {
+    final notification = message.notification;
+    if (notification == null) return;
+
+    _localNotifications.show(
+      id: notification.hashCode,
+      title: notification.title,
+      body: notification.body,
+      notificationDetails: NotificationDetails(
+        android: AndroidNotificationDetails(
+          _channel.id,
+          _channel.name,
+          channelDescription: _channel.description,
+          importance: Importance.high,
+          priority: Priority.high,
+        ),
+        iOS: const DarwinNotificationDetails(
+          presentAlert: true,
+          presentBadge: true,
+          presentSound: true,
+        ),
+      ),
+      payload: jsonEncode(message.data),
+    );
+  }
+
+  static void _onNotificationTap(NotificationResponse response) {
+    if (response.payload == null) return;
+    try {
+      final data = jsonDecode(response.payload!) as Map<String, dynamic>;
+      _navigateFromMessage(data);
+    } catch (_) {}
+  }
+
+  static void _onMessageOpenedApp(RemoteMessage message) {
+    _navigateFromMessage(message.data);
+  }
+
+  static void _navigateFromMessage(Map<String, dynamic> data) {
+    final sessionId = data['session_id'] as String?;
+    if (sessionId == null || _router == null) return;
+
+    final type = data['type'] as String?;
+    if (type == 'chat_message' || type == 'chat_request') {
+      _router!.push('/chat/session/$sessionId');
+    }
+  }
+}