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/router.dart

@@ -2,6 +2,7 @@ import 'dart:async';
 import 'package:flutter/material.dart';
 import 'package:go_router/go_router.dart';
 import 'core/auth/auth_bloc.dart';
+import 'features/splash/splash_screen.dart';
 import 'features/auth/screens/login_screen.dart';
 import 'features/auth/screens/otp_screen.dart';
 import 'features/home/home_screen.dart';
@@ -26,19 +27,26 @@ class _BlocRefreshNotifier extends ChangeNotifier {
 
 GoRouter buildRouter(AuthBloc authBloc) {
   return GoRouter(
-    initialLocation: '/login',
+    initialLocation: '/splash',
     refreshListenable: _BlocRefreshNotifier(authBloc),
     redirect: (context, state) {
       final authState = authBloc.state;
+      final isSplash = state.matchedLocation == '/splash';
       final isAuthRoute = state.matchedLocation.startsWith('/login') ||
           state.matchedLocation.startsWith('/otp');
 
-      if (authState is AuthLoading) return null;
-      if (authState is AuthAuthenticated) return isAuthRoute ? '/home' : null;
-      if (!isAuthRoute) return '/login';
+      // Show splash while loading
+      if (authState is AuthLoading) return isSplash ? null : '/splash';
+
+      if (authState is AuthAuthenticated) {
+        return (isSplash || isAuthRoute) ? '/home' : null;
+      }
+      if (!isAuthRoute && !isSplash) return '/login';
+      if (isSplash) return '/login';
       return null;
     },
     routes: [
+      GoRoute(path: '/splash', builder: (_, __) => const SplashScreen()),
       GoRoute(path: '/login', builder: (_, __) => const LoginScreen()),
       GoRoute(path: '/otp', builder: (context, state) => OtpScreen(phone: state.extra as String)),
       GoRoute(path: '/home', builder: (_, __) => const HomeScreen()),