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>

src/services/template_service.py

@@ -15,25 +15,25 @@ _TEMPLATES_FILENAME = "templates.json"
 BUILTIN_TEMPLATES: List[Template] = [
     Template(
         name="正方形模板",
-        x_ratio=0.34, y_ratio=0.22,
-        width_ratio=0.32, height_ratio=0.32,
+        x_ratio=0.40, y_ratio=0.40,
+        width_ratio=0.25, height_ratio=0.25,
         rotation=0.0, type="builtin",
     ),
     Template(
         name="纵向长方形模板",
-        x_ratio=0.35, y_ratio=0.16,
-        width_ratio=0.30, height_ratio=0.46,
+        x_ratio=0.435, y_ratio=0.385,
+        width_ratio=0.20, height_ratio=0.30,
         rotation=0.0, type="builtin",
     ),
     Template(
         name="横向长方形模板",
-        x_ratio=0.27, y_ratio=0.26,
-        width_ratio=0.46, height_ratio=0.30,
+        x_ratio=0.385, y_ratio=0.435,
+        width_ratio=0.30, height_ratio=0.20,
         rotation=0.0, type="builtin",
     ),
     Template(
         name="左胸小号模板",
-        x_ratio=0.50, y_ratio=0.22,
+        x_ratio=0.55, y_ratio=0.30,
         width_ratio=0.16, height_ratio=0.16,
         rotation=0.0, type="builtin",
     ),