Replace native Twilio Voice SDK with WebView-based softphone

Rewrites the mobile app from a native Twilio Voice SDK integration
(Android Telecom/ConnectionService) to a thin WebView shell that loads
a standalone browser phone page from WordPress. This eliminates the
buggy Android phone account registration, fixes frequent logouts by
using 7-day WP session cookies instead of JWT tokens, and maintains
all existing call features (dialpad, queues, hold, transfer, requeue,
recording, caller ID, agent status).

Server-side:
- Add class-twp-mobile-phone-page.php: standalone /twp-phone/ endpoint
  with mobile-optimized UI, dark mode, tab navigation, and Flutter
  WebView JS bridge
- Extend auth cookie to 7 days for phone agents
- Add WP AJAX handler for FCM token registration (cookie auth)

Flutter app (v2.0.0):
- Replace 18 native files with 5-file WebView shell
- Login via wp-login.php in WebView (auto-detect redirect on success)
- Full-screen WebView with auto microphone grant for WebRTC
- FCM push notifications preserved for queue alerts
- Remove: twilio_voice, dio, provider, JWT auth, SSE, native call UI

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

mobile/pubspec.yaml

@@ -1,7 +1,7 @@
 name: twp_softphone
-description: TWP Softphone - VoIP client for Twilio WordPress Plugin
+description: TWP Softphone - WebView client for Twilio WordPress Plugin
 publish_to: 'none'
-version: 1.0.0+1
+version: 2.0.0+6
 
 environment:
   sdk: ^3.5.0
@@ -9,21 +9,17 @@ environment:
 dependencies:
   flutter:
     sdk: flutter
-  twilio_voice: ^0.3.0
   firebase_core: ^3.0.0
   firebase_messaging: ^15.0.0
-  dio: ^5.4.0
   flutter_secure_storage: ^10.0.0
-  provider: ^6.1.0
   flutter_local_notifications: ^17.0.0
-  json_annotation: ^4.8.0
+  webview_flutter: ^4.10.0
+  webview_flutter_android: ^4.3.0
 
 dev_dependencies:
   flutter_test:
     sdk: flutter
   flutter_lints: ^4.0.0
-  build_runner: ^2.4.0
-  json_serializable: ^6.7.0
 
 flutter:
   uses-material-design: true