fix: keep print and re-apply selected template on preview switch

Switching the preview garment cleared the scene (print lost), and
re-loading a print used the canvas's hard-coded centred default instead
of the selected template.

- template_panel: add apply_current() to re-emit the selected template
  for the current garment/print sizes and sync _current_state.
- main_window: remember the current print path, restore it after a
  garment switch, and call apply_current() after garment/print changes
  so the selected template stays sticky. Fine-tuned items are unaffected.
- Also feed template_applied into export_panel.set_transform to keep the
  single-export transform in sync.

Docs: record the sticky-template rule (PRD 6.6).

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

docs/02-prd.md

@@ -163,6 +163,7 @@
 - 模板参数应基于衣服底图尺寸计算，避免只适配单一图片分辨率。
 - 模板的 `width_ratio` / `height_ratio` 定义衣服上的**目标框**，而非强制拉伸尺寸。已知印花原始尺寸时，印花按自身宽高比缩放后 contain 进目标框并居中，不被拉伸变形；未知印花尺寸时退化为铺满目标框。
 - 内置模板尺寸以贴近真实印花占比为准（例如正方形模板约为衣服宽度的 1/3），避免印花占据过大面积。
+- 选中的模板是**粘性**的：在预览区切换衣服或切换印花时，印花不应丢失，应按当前选中模板基于新的衣服/印花尺寸重新计算落位，而非回退到画布的写死默认值。已手动微调的项不受影响，仍使用其自身参数。
 
 ### 6.7 自定义模板
 

src/app/main_window.py

@@ -50,6 +50,7 @@ class MainWindow(QMainWindow):
         # Fine-tune state: track which queue item is currently being edited
         self._active_queue_item = None
         self._loading_queue_item = False  # suppress fine-tune marking during item load
+        self._current_print_path = None   # keep the print across garment switches
 
         root = QWidget()
         root.setObjectName("root")
@@ -277,11 +278,23 @@ class MainWindow(QMainWindow):
         self.template_panel.template_applied.connect(self.image_canvas.set_transform)
         self.template_panel.template_applied.connect(self.transform_panel.set_transform)
         self.template_panel.template_applied.connect(self.queue_panel.set_transform)
+        self.template_panel.template_applied.connect(self.export_panel.set_transform)
 
         # ── Export panel secondary buttons → template panel ────────────────
         self.export_panel.apply_as_template.connect(self.template_panel._save_as_template)
         self.export_panel.reset_to_template.connect(self.template_panel._reset_to_template)
 
+    def _load_print_preview(self, path):
+        """Load a print into the canvas and sync its native size to the template panel."""
+        self.image_canvas.load_print(path)
+        self.export_panel.set_print(path)
+        try:
+            from PIL import Image as _PilImage
+            with _PilImage.open(path) as img:
+                self.template_panel.set_print_size(img.width, img.height)
+        except Exception:
+            pass
+
     def _on_garment_preview(self, asset):
         """Load a garment from the asset list into the canvas for direct preview."""
         self._active_queue_item = None   # leave queue mode
@@ -298,23 +311,24 @@ class MainWindow(QMainWindow):
                 self.template_panel.set_garment_size(img.width, img.height)
         except Exception:
             pass
+        # load_garment clears the scene; restore the current print and re-lay it
+        # out with the selected template (instead of the canvas default).
+        if self._current_print_path:
+            self._load_print_preview(self._current_print_path)
+            self.template_panel.apply_current()
 
     def _on_print_preview(self, asset):
         """Load a print from the asset list into the canvas for direct preview."""
         self._active_queue_item = None
         if not asset:
+            self._current_print_path = None
             self.export_panel.set_print(None)
             return
-        path = str(asset.path)
-        self.image_canvas.load_print(path)
-        self.export_panel.set_print(path)
-        # Tell template panel the print dimensions for aspect-aware fitting
-        try:
-            from PIL import Image as _PilImage
-            with _PilImage.open(path) as img:
-                self.template_panel.set_print_size(img.width, img.height)
-        except Exception:
-            pass
+        self._current_print_path = str(asset.path)
+        self._load_print_preview(self._current_print_path)
+        # Apply the selected template so the print lands per the chosen layout,
+        # not the canvas's hard-coded centred default.
+        self.template_panel.apply_current()
 
     def _on_queue_item_activated(self, item):
         """Load a queue item into the canvas and arm fine-tune tracking."""

src/app/widgets/template_panel.py

@@ -138,6 +138,19 @@ class TemplatePanel(QWidget):
             self._print_w or None, self._print_h or None,
         )
 
+    def apply_current(self):
+        """Re-apply the currently selected template using current garment/print sizes.
+
+        Lets the selected template stay 'sticky': callers invoke this after the
+        preview garment or print changes so the layout is recomputed from the
+        template instead of falling back to the canvas default.
+        """
+        t = self._current_template()
+        if t and self._garment_w > 0 and self._garment_h > 0:
+            state = self._state_for_template(t)
+            self._current_state = state
+            self.template_applied.emit(state)
+
     def set_transform(self, state: TransformState):
         """Store current print transform (used when saving a template)."""
         self._current_state = state