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/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));