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>

client_app/lib/router.dart

@@ -7,6 +7,7 @@ import 'features/auth/screens/display_name_screen.dart';
 import 'features/auth/screens/register_screen.dart';
 import 'features/auth/screens/otp_screen.dart';
 import 'features/auth/screens/force_register_screen.dart';
+import 'features/splash/splash_screen.dart';
 import 'features/home/home_screen.dart';
 import 'features/chat/screens/searching_screen.dart';
 import 'features/chat/screens/bestie_found_screen.dart';
@@ -32,24 +33,27 @@ class _BlocRefreshNotifier extends ChangeNotifier {
 
 GoRouter buildRouter(AuthBloc authBloc) {
   return GoRouter(
-    initialLocation: '/welcome',
+    initialLocation: '/splash',
     refreshListenable: _BlocRefreshNotifier(authBloc),
     redirect: (context, state) {
       final authState = authBloc.state;
+      final isSplash = state.matchedLocation == '/splash';
       final isAuthRoute = state.matchedLocation.startsWith('/auth') ||
           state.matchedLocation == '/welcome';
 
-      // Don't redirect while loading — stay on current screen
-      if (authState is AuthLoading) return null;
+      // Show splash while loading
+      if (authState is AuthLoading) return isSplash ? null : '/splash';
 
       if (authState is AuthAuthenticated || authState is AuthAnonymous) {
-        return isAuthRoute ? '/home' : null;
+        return (isSplash || isAuthRoute) ? '/home' : null;
       }
       if (authState is AuthForceRegister) return '/auth/force-register';
-      if (!isAuthRoute) return '/welcome';
+      if (!isAuthRoute && !isSplash) return '/welcome';
+      if (isSplash) return '/welcome';
       return null;
     },
     routes: [
+      GoRoute(path: '/splash', builder: (_, __) => const SplashScreen()),
       GoRoute(path: '/welcome', builder: (_, __) => const WelcomeScreen()),
       GoRoute(path: '/auth/display-name', builder: (_, __) => const DisplayNameScreen()),
       GoRoute(path: '/auth/register', builder: (_, __) => const RegisterScreen()),
@@ -66,10 +70,13 @@ GoRouter buildRouter(AuthBloc authBloc) {
       }),
       GoRoute(path: '/chat/no-bestie', builder: (_, __) => const NoBestieScreen()),
       GoRoute(path: '/chat/session/:sessionId', builder: (context, state) {
-        final extra = state.extra as Map<String, dynamic>?;
+        final extra = state.extra;
+        final mitraName = extra is String
+            ? extra
+            : (extra is Map<String, dynamic> ? extra['mitraName'] as String? : null);
         return ChatScreen(
           sessionId: state.pathParameters['sessionId']!,
-          mitraName: extra?['mitraName'] as String? ?? 'Bestie',
+          mitraName: mitraName ?? 'Bestie',
         );
       }),
       GoRoute(path: '/chat/history', builder: (_, __) => const ChatHistoryScreen()),