feat: enable Windows app internet access and improve server discovery

frontend/lib/core/network/api_constants.dart

@@ -2,8 +2,8 @@ class ApiConstants {
   static const int port = 18484;
 
   // Runtime-mutable base URL — set by ServerDiscoveryService before first use.
-  // Default: localhost for Windows desktop, Android will discover via LAN scan.
-  static String _baseUrl = 'http://127.0.0.1:$port/api';
+  // Default: remote internet/configured server IP or domain.
+  static String _baseUrl = 'http://198.252.107.197:$port/api';
 
   static String get baseUrl => _baseUrl;
 

frontend/lib/core/network/server_discovery.dart

@@ -33,9 +33,12 @@ class ServerDiscoveryService {
 
   static Future<bool> ping(String baseUrl) async {
     try {
+      final rootUrl = baseUrl
+          .replaceAll(RegExp(r'/+$'), '')
+          .replaceAll(RegExp(r'/api$'), '');
       final dio = Dio(
         BaseOptions(
-          baseUrl: baseUrl,
+          baseUrl: rootUrl,
           connectTimeout: _pingTimeout,
           receiveTimeout: _pingTimeout,
         ),