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/app/widgets/image_list_panel.py

@@ -155,12 +155,14 @@ class _AssetPanel(QWidget):
 
     preview_changed = Signal(object)   # ImageAsset | None
     assets_changed = Signal(list)      # List[ImageAsset]
+    folder_opened = Signal(str)        # chosen folder path (for persistence)
 
     def __init__(self, title: str, open_text: str, parent=None):
         super().__init__(parent)
         self._assets: List[ImageAsset] = []
         self._root_folder: Optional[Path] = None
         self._current_preview_row: int = -1
+        self._start_dir: str = ""       # where the folder picker opens
         self._setup_ui(title, open_text)
 
     # ── UI construction ────────────────────────────────────────────────
@@ -238,10 +240,16 @@ class _AssetPanel(QWidget):
 
     # ── folder scanning ────────────────────────────────────────────────
 
+    def set_start_dir(self, path: str):
+        """Set where the folder picker opens (restored last-used folder)."""
+        self._start_dir = path or ""
+
     def _open_folder(self):
-        folder = QFileDialog.getExistingDirectory(self, "选择文件夹")
+        folder = QFileDialog.getExistingDirectory(self, "选择文件夹", self._start_dir)
         if not folder:
             return
+        self._start_dir = folder
+        self.folder_opened.emit(folder)
         self._root_folder = Path(folder)
         try:
             self._assets = scan_image_folder(self._root_folder)
@@ -251,6 +259,9 @@ class _AssetPanel(QWidget):
         logger.info("Loaded %d asset(s) from %s", len(self._assets), folder)
         self._apply_sort()
         self.assets_changed.emit(list(self._assets))
+        # Auto-preview the first image so the canvas isn't empty after loading
+        if self._assets:
+            self._select_preview_row(0)
 
     # ── sorting & list rebuild ─────────────────────────────────────────
 
@@ -284,7 +295,12 @@ class _AssetPanel(QWidget):
     # ── interaction ────────────────────────────────────────────────────
 
     def _on_item_clicked(self, item: QListWidgetItem):
-        row = self._list.row(item)
+        self._select_preview_row(self._list.row(item))
+
+    def _select_preview_row(self, row: int):
+        """Highlight the row as the current preview and emit preview_changed."""
+        if not (0 <= row < self._list.count()):
+            return
 
         # Clear previous preview highlight
         if 0 <= self._current_preview_row < self._list.count():
@@ -295,7 +311,7 @@ class _AssetPanel(QWidget):
                     w.set_preview_selected(False)
 
         self._current_preview_row = row
-        widget = self._list.itemWidget(item)
+        widget = self._list.itemWidget(self._list.item(row))
         if widget:
             widget.set_preview_selected(True)
             self.preview_changed.emit(widget.asset)
@@ -357,6 +373,8 @@ class ImageListPanel(QWidget):
     print_preview_changed = Signal(object)     # ImageAsset | None
     garments_changed = Signal(list)            # List[ImageAsset]
     prints_changed = Signal(list)              # List[ImageAsset]
+    garment_folder_opened = Signal(str)        # last-used garment folder
+    print_folder_opened = Signal(str)          # last-used print folder
 
     def __init__(self, parent=None):
         super().__init__(parent)
@@ -378,6 +396,8 @@ class ImageListPanel(QWidget):
         self._print_panel.preview_changed.connect(self.print_preview_changed)
         self._garment_panel.assets_changed.connect(self.garments_changed)
         self._print_panel.assets_changed.connect(self.prints_changed)
+        self._garment_panel.folder_opened.connect(self.garment_folder_opened)
+        self._print_panel.folder_opened.connect(self.print_folder_opened)
 
         splitter.addWidget(self._garment_panel)
         splitter.addWidget(self._print_panel)
@@ -456,3 +476,9 @@ class ImageListPanel(QWidget):
 
     def get_prints(self) -> List[ImageAsset]:
         return self._print_panel.get_assets()
+
+    def set_garment_start_dir(self, path: str):
+        self._garment_panel.set_start_dir(path)
+
+    def set_print_start_dir(self, path: str):
+        self._print_panel.set_start_dir(path)