fix(mackup): update backup path and enhance configuration handling

- Corrected the backup path in the `_in_backup` function to reflect the new structure under `mackup/`.
- Improved error handling in `ssr::mackup::configure` to skip restore if the configuration file is not updated, providing user feedback.
- Updated `ssr::sync` to conditionally execute backup based on configuration validity, enhancing robustness.
- Revised documentation in configuration examples to clarify the new Mackup directory structure and its implications for users.

bin/ssr-mackup

@@ -71,7 +71,7 @@ cmd_scan() {
         done <<< "$declared"
         return 1
     }
-    _in_backup() { [[ -n "$mackup_dir" && -e "$mackup_dir/$1" ]]; }
+    _in_backup() { [[ -n "$mackup_dir" && -e "$mackup_dir/mackup/$1" ]]; }
 
     # -----------------------------------------------------------------------
     # Collect candidate items — same logic as generate_custom_cfg.