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/features/chat/screens/chat_transcript_screen.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_bloc/flutter_bloc.dart';
 import '../../../core/api/api_client.dart';
+import '../../../core/constants.dart';
 
 class ChatTranscriptScreen extends StatefulWidget {
   final String sessionId;
@@ -47,7 +48,7 @@ class _ChatTranscriptScreenState extends State<ChatTranscriptScreen> {
               padding: const EdgeInsets.all(16),
               children: [
                 ..._messages.map((m) {
-                  final isMe = m['sender_type'] == 'customer';
+                  final isMe = m['sender_type'] == UserType.customer;
                   final time = DateTime.parse(m['created_at'] as String).toLocal();
                   return Align(
                     alignment: isMe ? Alignment.centerRight : Alignment.centerLeft,
@@ -79,7 +80,7 @@ class _ChatTranscriptScreenState extends State<ChatTranscriptScreen> {
                   const SizedBox(height: 8),
                   ..._closures.map((c) => Card(
                     child: ListTile(
-                      title: Text(c['user_type'] == 'customer' ? 'Kamu' : 'Bestie'),
+                      title: Text(c['user_type'] == UserType.customer ? 'Kamu' : 'Bestie'),
                       subtitle: Text(c['message'] as String),
                     ),
                   )),