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/05-project-architecture.md

@@ -240,7 +240,7 @@ BatchResult
 - 读取和保存应用配置。
 - 管理默认配置。
 - 在配置损坏或缺失时提供安全默认值。
-- 持久化用户偏好：输出设置、上次选择的模板（`last_template`）、上次选择的批量模式（`last_batch_mode`）等。
+- 持久化用户偏好：输出设置、上次的衣服/印花文件夹（`last_garment_dir`/`last_print_dir`）、上次选择的模板（`last_template`）、上次选择的批量模式（`last_batch_mode`）等。
 
 由主窗口集中使用：启动时加载一次并把初值分发给各面板，面板选择变化时「改一次存一次」回写。各 UI 控件不直接读写配置文件，避免分散解析。