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