Apply Settings changes to the live BackendAPI without restart

Settings._save wrote new base_url/api_key to prefs but the BackendAPI
singleton kept the Dio it built at first access, so URL/key changes only
took effect after an app restart. Expose reconfigureFromPrefs() on
BackendAPI and call it from Settings._save so subsequent requests pick up
the new config immediately.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/lib/settings/settings.dart

@@ -71,6 +71,7 @@ class _SettingsState extends State<Settings> {
     if (!_formKey.currentState!.validate()) return;
     PrefUtil.setValue('base_url', _baseUrlController.text);
     PrefUtil.setValue('api_key', XOR().xorEncode(_apiKeyController.text));
+    BackendAPI.instance.reconfigureFromPrefs();
     context.read<AppState>().setTheme(_selectedTheme);
     UISnackbars.showSuccess(context, 'Settings saved successfully');
   }

frontend/lib/utils/oott_api.dart

@@ -57,6 +57,10 @@ class BackendAPI {
   static BackendAPI get instance => _instance;
 
   BackendAPI._internal() {
+    reconfigureFromPrefs();
+  }
+
+  void reconfigureFromPrefs() {
     _baseUrl =
         PrefUtil.getValue("base_url", "http://localhost:3000/api") as String;
     _apiKey = XOR().xorDecode(PrefUtil.getValue("api_key", "") as String);