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>

includes/class-twp-core.php

@@ -39,6 +39,7 @@ class TWP_Core {
         require_once TWP_PLUGIN_DIR . 'includes/class-twp-mobile-api.php';
         require_once TWP_PLUGIN_DIR . 'includes/class-twp-mobile-sse.php';
         require_once TWP_PLUGIN_DIR . 'includes/class-twp-fcm.php';
+        require_once TWP_PLUGIN_DIR . 'includes/class-twp-mobile-phone-page.php';
         require_once TWP_PLUGIN_DIR . 'includes/class-twp-auto-updater.php';
 
         // Feature classes
@@ -254,7 +255,10 @@ class TWP_Core {
         
         // Initialize Shortcodes
         TWP_Shortcodes::init();
-        
+
+        // Initialize standalone mobile phone page (/twp-phone/)
+        new TWP_Mobile_Phone_Page();
+
         // Scheduled events
         $scheduler = new TWP_Scheduler();
         $this->loader->add_action('twp_check_schedules', $scheduler, 'check_active_schedules');