feat: LAN update check on startup with notify banner (stage 2)

Read manifest.json from the configured update_source and show a dismissable
banner when a newer version is available. Notify-only — no install yet.

- services/update_service.py: version compare + check_for_update (pure, tested)
- config: add update_source key (empty = no check)
- main_window: top banner, background-thread check; open folder via os.startfile
  (QDesktopServices.openUrl mishandles file:// folder URLs — ShellExecute err 2)
- tests: +15 covering version compare and check_for_update branches
- docs 02/05/10 + tasks 17.16

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

docs/02-prd.md

@@ -330,7 +330,8 @@ CMBot/
   "last_garment_dir": "",
   "last_print_dir": "",
   "last_template": "正方形模板",
-  "last_batch_mode": "full_combo"
+  "last_batch_mode": "full_combo",
+  "update_source": ""
 }
 ```
 
@@ -339,6 +340,7 @@ CMBot/
 - `last_garment_dir` / `last_print_dir`：上次打开的衣服 / 印花文件夹路径，用于让文件夹对话框定位到上次位置。
 - `last_template`：上次选中的模板名称，用于启动恢复。
 - `last_batch_mode`：上次选择的批量模式（取 `BatchMode` 枚举值，如 `full_combo` / `many_garments` / `many_prints` / `one_to_one`）。
+- `update_source`：局域网更新源目录（含 `manifest.json`）。为空时不做更新检查。详见 `docs/10-lan-update.md`。
 - 偏好的读取、分发与「改一次存一次」由主窗口集中处理，UI 控件不直接读写配置文件。
 
 ## 10. 验收标准