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>

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