fix(update): preserve .git on release channel switch, use git checkout tag

Previously, switching to the release channel would delete .git/ and
fall back to a ZIP download. This broke update.bat, manual git
commands, and CM git-based update detection.

Now the release path uses git checkout <latest-tag> when .git exists,
and only falls back to ZIP when .git is absent (CM CNR installs).
.git is never deleted - the ZIP→nightly path remains a one-way
upgrade via _init_git_repo.

Also updates locale strings (en, zh-CN, zh-TW, ja) to remove the
now-inaccurate "remove the Git repository" wording.

locales/zh-CN.json

@@ -1782,7 +1782,7 @@
       "nightlyTitle": "切换到 Nightly",
       "nightlyMessage": "切换到 Nightly 将初始化 Git 仓库并跟踪 main 分支的最新提交。更新更频繁但可能不稳定，可随时切回稳定版。",
       "releaseTitle": "切换到稳定版",
-      "releaseMessage": "切换到稳定版将移除 Git 仓库并安装最新的稳定发布版本，后续仅使用稳定版更新。",
+      "releaseMessage": "切换到稳定版将检出最新的发布标签。Git 仓库和您的设置均会保留。可随时切换回每日构建版。",
       "switching": "正在切换到 {channel} 频道...",
       "completed": "已切换到 {channel} 频道",
       "failed": "切换频道失败"