fix: resolve connectivity issues for Android and Web builds

- Android: Added INTERNET permission and enabled cleartext traffic in AndroidManifest.xml.
- Web: Implemented and registered CORSMiddleware in backend to allow cross-origin requests.
- Flutter: Updated SettingsProvider to use 10.0.2.2 as default for Android Emulator for easier local testing.

frontend/lib/providers/settings_provider.dart

@@ -1,9 +1,14 @@
 import 'package:flutter/material.dart';
 import 'package:shared_preferences/shared_preferences.dart';
+import 'package:flutter/foundation.dart';
+
 class SettingsProvider with ChangeNotifier {
-  // Default server address for local development.
-  // Using 10.0.2.2 for Android emulator or localhost for Desktop.
-  String _baseUrl = "http://localhost:8080";
+  // Use 10.0.2.2 for Android emulator to access host's localhost
+  static String get _defaultUrl => (defaultTargetPlatform == TargetPlatform.android && !kIsWeb) 
+      ? "http://10.0.2.2:8080" 
+      : "http://localhost:8080";
+
+  String _baseUrl = _defaultUrl;
   Color _themeColor = Colors.blue;
 
   String get baseUrl => _baseUrl;