Add wake lock and device keepalive for reliable mobile calls

Prevents screen sleep during calls and auto-reconnects the Twilio device
after app backgrounding. JS handles Screen Wake Lock API and 30s keepalive
polling; Flutter coordinates via WakelockPlus and synthetic visibilitychange
events on app resume.

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

mobile/lib/screens/phone_screen.dart

@@ -1,5 +1,6 @@
 import 'package:flutter/material.dart';
 import 'package:permission_handler/permission_handler.dart';
+import 'package:wakelock_plus/wakelock_plus.dart';
 import 'package:webview_flutter/webview_flutter.dart';
 import 'package:webview_flutter_android/webview_flutter_android.dart';
 import '../services/push_notification_service.dart';
@@ -52,10 +53,27 @@ class _PhoneScreenState extends State<PhoneScreen> with WidgetsBindingObserver {
     }
     _initWebView();
     _initPush();
+    WakelockPlus.enable();
+    debugPrint('TWP: Wake lock enabled');
+  }
+
+  @override
+  void didChangeAppLifecycleState(AppLifecycleState state) {
+    if (state == AppLifecycleState.resumed) {
+      debugPrint('TWP: App resumed — re-enabling wake lock');
+      WakelockPlus.enable();
+      // Dispatch synthetic visibilitychange to trigger JS-side recovery
+      _controller.runJavaScript(
+        'document.dispatchEvent(new Event("visibilitychange"));'
+        'Object.defineProperty(document, "hidden", {value: false, configurable: true});',
+      );
+    }
   }
 
   @override
   void dispose() {
+    WakelockPlus.disable();
+    debugPrint('TWP: Wake lock disabled');
     WidgetsBinding.instance.removeObserver(this);
     super.dispose();
   }