Phase 3 scaffold: chat engine (WebSocket, FCM, pricing, timer, extension, history)

- Backend: WebSocket plugin, chat/pricing/timer/extension/closure/notification services
- Client app: ChatBloc, pricing dialog, chat screen with message status, extension/goodbye flow, history
- Mitra app: MitraChatBloc, ExtensionBloc, chat screen, extension accept/reject, history
- Control center: free trial, extension timeout, early end config toggles
- DB migration: chat_messages, session_closures, session_extensions, customer_transactions tables

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

client_app/macos/Flutter/GeneratedPluginRegistrant.swift

@@ -7,6 +7,7 @@ import Foundation
 
 import firebase_auth
 import firebase_core
+import firebase_messaging
 import google_sign_in_ios
 import shared_preferences_foundation
 import sign_in_with_apple
@@ -14,6 +15,7 @@ import sign_in_with_apple
 func RegisterGeneratedPlugins(registry: FlutterPluginRegistry) {
   FLTFirebaseAuthPlugin.register(with: registry.registrar(forPlugin: "FLTFirebaseAuthPlugin"))
   FLTFirebaseCorePlugin.register(with: registry.registrar(forPlugin: "FLTFirebaseCorePlugin"))
+  FLTFirebaseMessagingPlugin.register(with: registry.registrar(forPlugin: "FLTFirebaseMessagingPlugin"))
   FLTGoogleSignInPlugin.register(with: registry.registrar(forPlugin: "FLTGoogleSignInPlugin"))
   SharedPreferencesPlugin.register(with: registry.registrar(forPlugin: "SharedPreferencesPlugin"))
   SignInWithApplePlugin.register(with: registry.registrar(forPlugin: "SignInWithApplePlugin"))