fix(settings): save all config atomically and strip trailing slash from API URL

- Save API URL and sound paths in a single load-modify-write cycle to
  prevent race conditions that wiped previously saved config
- Strip trailing slashes from API URL to avoid double-slash 404 errors
- Make AppConfigService.saveConfig public for batch config writes

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/pages/settings_page.dart

@@ -57,14 +57,27 @@ class _SettingsPageState extends State<SettingsPage> {
   }
 
   Future<void> _saveUrl() async {
-    final url = _controller.text.trim();
+    final url = _controller.text.trim().replaceAll(RegExp(r'/+$'), '');
     if (url.isEmpty) {
       _showSnackBar('请输入 API 地址', isError: true);
       return;
     }
     setState(() => _saving = true);
+    // Load config once, apply all changes, then save once to avoid race conditions
     final configService = AppConfigService();
-    await configService.setString('api_url', url);
+    final config = await configService.loadConfig();
+    config['api_url'] = url;
+    if (_successPath != null) config['sound_success'] = _successPath;
+    else config.remove('sound_success');
+    if (_failurePath != null) config['sound_failure'] = _failurePath;
+    else config.remove('sound_failure');
+    if (_beepPath != null) config['sound_beep'] = _beepPath;
+    else config.remove('sound_beep');
+    if (_errorPath != null) config['sound_error'] = _errorPath;
+    else config.remove('sound_error');
+    if (_alertPath != null) config['sound_alert'] = _alertPath;
+    else config.remove('sound_alert');
+    await configService.saveConfig(config);
     setState(() => _saving = false);
     if (mounted) {
       _showSnackBar('设置已保存');
@@ -91,18 +104,6 @@ class _SettingsPageState extends State<SettingsPage> {
     );
     if (result != null && result.files.single.path != null) {
       final path = result.files.single.path!;
-      switch (key) {
-        case 'success':
-          await _soundService.setSuccessPath(path);
-        case 'failure':
-          await _soundService.setFailurePath(path);
-        case 'beep':
-          await _soundService.setBeepPath(path);
-        case 'error':
-          await _soundService.setErrorPath(path);
-        case 'alert':
-          await _soundService.setAlertPath(path);
-      }
       setState(() {
         switch (key) {
           case 'success':
@@ -121,18 +122,6 @@ class _SettingsPageState extends State<SettingsPage> {
   }
 
   Future<void> _clearSound(String key) async {
-    switch (key) {
-      case 'success':
-        await _soundService.setSuccessPath(null);
-      case 'failure':
-        await _soundService.setFailurePath(null);
-      case 'beep':
-        await _soundService.setBeepPath(null);
-      case 'error':
-        await _soundService.setErrorPath(null);
-      case 'alert':
-        await _soundService.setAlertPath(null);
-    }
     setState(() {
       switch (key) {
         case 'success':

lib/services/app_config_service.dart

@@ -38,11 +38,11 @@ class AppConfigService {
   Future<void> setString(String key, String value) async {
     final config = await loadConfig();
     config[key.toString()] = value.toString();
-    await _saveConfig(config);
+    await saveConfig(config);
   }
 
   /// 完整写入 JSON 文件
-  Future<void> _saveConfig(Map<String, dynamic> config) async {
+  Future<void> saveConfig(Map<String, dynamic> config) async {
     try {
       final file = File(await _filePath);
       await file.writeAsString(jsonEncode(config));