refactor(backend): migrate import/export to use SQL backup

- Reimplement export_config_to_file to use database.export_sql
- Reimplement import_config_from_file to use database.import_sql
- Add sync_current_from_db to sync live configs after import
- Add settings database binding on app initialization
- Remove deprecated JSON-based config import logic

src-tauri/src/lib.rs

@@ -570,6 +570,7 @@ pub fn run() {
                 }
             }
 
+            crate::settings::bind_db(db.clone());
             let app_state = AppState::new(db);
 
             // 检查是否需要首次导入（数据库为空）