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/constants.dart

@@ -0,0 +1,83 @@
+/// User types
+class UserType {
+  static const customer = 'customer';
+  static const mitra = 'mitra';
+  UserType._();
+}
+
+/// Chat session statuses
+class SessionStatus {
+  static const searching = 'searching';
+  static const pendingAcceptance = 'pending_acceptance';
+  static const pendingPayment = 'pending_payment';
+  static const active = 'active';
+  static const extending = 'extending';
+  static const closing = 'closing';
+  static const completed = 'completed';
+  static const cancelled = 'cancelled';
+  static const expired = 'expired';
+  SessionStatus._();
+}
+
+/// Chat message statuses
+class MessageStatus {
+  static const sent = 'sent';
+  static const delivered = 'delivered';
+  static const read = 'read';
+  MessageStatus._();
+}
+
+/// Chat message types
+class MessageType {
+  static const text = 'text';
+  MessageType._();
+}
+
+/// Session extension statuses
+class ExtensionStatus {
+  static const pending = 'pending';
+  static const accepted = 'accepted';
+  static const rejected = 'rejected';
+  static const timeout = 'timeout';
+  ExtensionStatus._();
+}
+
+/// WebSocket message types
+class WsMessage {
+  // Auth
+  static const auth = 'auth';
+  static const authOk = 'auth_ok';
+  static const error = 'error';
+
+  // Chat
+  static const message = 'message';
+  static const messageAck = 'message_ack';
+  static const messageStatus = 'message_status';
+  static const typing = 'typing';
+
+  // Pairing
+  static const chatRequest = 'chat_request';
+  static const chatRequestClosed = 'chat_request_closed';
+  static const paired = 'paired';
+
+  // Session lifecycle
+  static const sessionTimer = 'session_timer';
+  static const sessionExpired = 'session_expired';
+  static const sessionClosing = 'session_closing';
+  static const sessionCompleted = 'session_completed';
+  static const sessionPaused = 'session_paused';
+  static const sessionResumed = 'session_resumed';
+
+  // Extension
+  static const extensionRequest = 'extension_request';
+  static const extensionResponse = 'extension_response';
+
+  // Delivery
+  static const delivered = 'delivered';
+  static const read = 'read';
+
+  // Early end
+  static const earlyEnd = 'early_end';
+
+  WsMessage._();
+}