feat(flutter): 增加 debug 运行时服务器地址设置

lib/core/config/app_config.dart

@@ -15,18 +15,28 @@ class AppConfig {
   /// `ipconfig getifaddr en0` 重新确认并改这里；
   /// 也可以换成 Bonjour 名字 `http://<LocalHostName>.local:8081`（本机是
   /// `jiangdeMacBook-Air.local`），换网络不用改，但部分办公网会拦组播解析。
-  static const String devHostBaseUrl =
-      'http://jiangdeMacBook-Air.local:8081';
+  static const String devHostBaseUrl = 'http://jiangdeMacBook-Air.local:8081';
 
   static const String _loopbackBaseUrl = 'http://127.0.0.1:8081';
 
   static const String _overrideBaseUrl = String.fromEnvironment('API_BASE_URL');
 
-  static String get apiBaseUrl =>
-      _overrideBaseUrl.isNotEmpty ? _overrideBaseUrl : _defaultBaseUrl;
+  static String _runtimeBaseUrl = '';
 
-  static final String _defaultBaseUrl =
-      _runsOnDevMachine ? _loopbackBaseUrl : devHostBaseUrl;
+  /// Build-time server URL supplied through `--dart-define=API_BASE_URL`.
+  static String get compiledBaseUrl => _overrideBaseUrl;
+
+  /// Existing platform-aware fallback used when no runtime or build override exists.
+  static String get defaultBaseUrl => _defaultBaseUrl;
+
+  /// Effective server URL, including the debug-only runtime override.
+  static String get apiBaseUrl => _runtimeBaseUrl.isNotEmpty
+      ? _runtimeBaseUrl
+      : (compiledBaseUrl.isNotEmpty ? compiledBaseUrl : defaultBaseUrl);
+
+  static final String _defaultBaseUrl = _runsOnDevMachine
+      ? _loopbackBaseUrl
+      : devHostBaseUrl;
 
   /// 模拟器、桌面端与开发机同机，环回地址可用；iOS 真机不满足。
   /// Android 没有等价的探测方式，统一按真机处理（模拟器用
@@ -39,6 +49,16 @@ class AppConfig {
       (defaultTargetPlatform == TargetPlatform.iOS &&
           Platform.environment.containsKey('SIMULATOR_DEVICE_NAME'));
 
+  /// Applies an already normalized runtime URL. Passing null clears it.
+  static void applyRuntimeBaseUrl(String? value) {
+    _runtimeBaseUrl = value ?? '';
+  }
+
+  /// Clears the in-memory runtime override, mainly for tests and restarts.
+  static void clearRuntimeBaseUrl() {
+    _runtimeBaseUrl = '';
+  }
+
   static const Duration requestTimeout = Duration(seconds: 8);
 
   /// 后端返回的是相对路径（/files/...），这里补上服务地址。