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/hooks/useProviderActions.ts

@@ -134,9 +134,20 @@ export function useProviderActions(activeApp: AppId) {
   const switchProvider = useCallback(
     async (provider: Provider) => {
       try {
-        await switchProviderMutation.mutateAsync(provider.id);
+        const result = await switchProviderMutation.mutateAsync(provider.id);
         await syncClaudePlugin(provider);
 
+        // Show backfill warning if present
+        if (result?.warnings?.length) {
+          toast.warning(
+            t("notifications.backfillWarning", {
+              defaultValue:
+                "切换成功，但旧供应商配置回填失败，您手动修改的配置可能未保存",
+            }),
+            { duration: 5000 },
+          );
+        }
+
         // 根据供应商类型显示不同的成功提示
         if (
           activeApp === "claude" &&