Add TWP Softphone Flutter app and complete mobile backend API

Backend: Add /voice/token endpoint with AccessToken + VoiceGrant for
mobile VoIP, implement unhold_call() with call leg detection, wire FCM
push notifications into call queue and webhook missed call handlers,
add data-only FCM message support for Android background wake, and add
Twilio API Key / Push Credential settings fields.

Flutter app: Full softphone with Twilio Voice SDK integration, JWT auth
with auto-refresh, SSE real-time queue updates, FCM push notifications,
Material 3 UI with dashboard, active call screen, dialpad, and call
controls (mute/speaker/hold/transfer).

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

mobile/pubspec.yaml

@@ -0,0 +1,29 @@
+name: twp_softphone
+description: TWP Softphone - VoIP client for Twilio WordPress Plugin
+publish_to: 'none'
+version: 1.0.0+1
+
+environment:
+  sdk: ^3.5.0
+
+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
+
+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