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/test/webview_app_test.dart

@@ -0,0 +1,40 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:twp_softphone/app.dart';
+import 'package:twp_softphone/screens/login_screen.dart';
+
+void main() {
+  group('TwpSoftphoneApp', () {
+    testWidgets('shows loading indicator on startup', (tester) async {
+      await tester.pumpWidget(const TwpSoftphoneApp());
+      expect(find.byType(TwpSoftphoneApp), findsOneWidget);
+      expect(find.bySubtype<CircularProgressIndicator>(), findsOneWidget);
+    });
+  });
+
+  group('LoginScreen', () {
+    testWidgets('renders server URL field and connect button', (tester) async {
+      await tester.pumpWidget(
+        MaterialApp(
+          home: LoginScreen(onLoginSuccess: (_) {}),
+        ),
+      );
+      await tester.pump(const Duration(milliseconds: 100));
+      expect(find.text('Server URL'), findsOneWidget);
+      expect(find.text('Connect'), findsOneWidget);
+      expect(find.text('TWP Softphone'), findsOneWidget);
+    });
+
+    testWidgets('validates empty server URL on submit', (tester) async {
+      await tester.pumpWidget(
+        MaterialApp(
+          home: LoginScreen(onLoginSuccess: (_) {}),
+        ),
+      );
+      await tester.pump(const Duration(milliseconds: 100));
+      await tester.tap(find.text('Connect'));
+      await tester.pump();
+      expect(find.text('Required'), findsOneWidget);
+    });
+  });
+}