feat(backup): add pre-migration backup, periodic backup, backfill warning, and backup management UI

Four improvements to the database backup mechanism:

1. Auto backup before schema migration - creates a snapshot when
   upgrading from an older database version, providing a safety net
   beyond the existing SAVEPOINT rollback mechanism.

2. Periodic startup backup - checks on app launch whether the latest
   backup is older than 24 hours and creates a new one if needed,
   ensuring all users have recent backups regardless of usage patterns.

3. Backfill failure notification - switch now returns SwitchResult with
   warnings instead of silently ignoring backfill errors, so users are
   informed when their manual config changes may not have been saved.

4. Backup management UI - new BackupListSection in Settings > Data
   Management showing all backup snapshots with restore capability,
   including a confirmation dialog and automatic safety backup before
   restore.

src-tauri/src/lib.rs

@@ -768,6 +768,11 @@ pub fn run() {
 
                 // 检查 settings 表中的代理状态，自动恢复代理服务
                 restore_proxy_state_on_startup(&state).await;
+
+                // Periodic backup check
+                if let Err(e) = state.db.periodic_backup_if_needed() {
+                    log::warn!("Periodic backup failed on startup: {e}");
+                }
             });
 
             // Linux: 禁用 WebKitGTK 硬件加速，防止 EGL 初始化失败导致白屏
@@ -896,6 +901,8 @@ pub fn run() {
             commands::save_file_dialog,
             commands::open_file_dialog,
             commands::open_zip_file_dialog,
+            commands::list_db_backups,
+            commands::restore_db_backup,
             commands::sync_current_providers_live,
             // Deep link import
             commands::parse_deeplink,