feat: auto-preview, timestamped/grouped output, folder memory; fix batch format

- Auto-preview the first image after a folder loads (both panels), so the
  canvas isn't empty (§17.8).
- Group exports as <output_dir>/<timestamp>/<print_stem>/<garment_stem>.<ext>;
  one timestamp per export run, shared across the batch (§17.9).
- Remember the last garment/print folders and reopen the picker there;
  persisted via config, wired through main_window (§17.10).
- Shrink the 3 box templates further and lower them (chest placement).
- Fix: batch export ignored the chosen format/quality/output dir because the
  queue never received ExportOptions. Export panel now emits
  export_options_changed → queue.set_export_options (+ initial sync) (§17.11).

Docs: PRD 6.1/6.8/9, architecture 4.12, UI design 5.1, tasks.md 17.8–17.11.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/07-ui-design.md

@@ -100,6 +100,7 @@
 交互规则：
 
 - 选择文件夹后递归导入该文件夹及所有子文件夹中的支持格式图片。
+- 加载文件夹后默认选中并预览第一张（按当前排序）；空文件夹则不选。
 - 支持全选和取消全选。
 - 支持单张勾选和取消勾选。
 - 支持点击某张衣服图片作为当前预览底图。