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/en.json

@@ -1782,7 +1782,7 @@
       "nightlyTitle": "Switch to Nightly Channel",
       "nightlyMessage": "Switching to Nightly will initialize a Git repository and track the latest main branch commits. Updates will be more frequent but may be unstable. You can switch back to Release at any time.",
       "releaseTitle": "Switch to Release Channel",
-      "releaseMessage": "Switching to Release will remove the Git repository and install the latest stable release. Future updates will use stable releases only.",
+      "releaseMessage": "Switching to Release will checkout the latest stable release tag. Your Git repository and settings are preserved. You can switch back to Nightly at any time.",
       "switching": "Switching to {channel} channel...",
       "completed": "Successfully switched to {channel} channel",
       "failed": "Failed to switch channel"

locales/ja.json

@@ -1782,7 +1782,7 @@
       "nightlyTitle": "ナイトリーチャンネルに切り替え",
       "nightlyMessage": "ナイトリーに切り替えると、Gitリポジトリが初期化され、mainブランチの最新コミットを追跡します。更新頻度は高くなりますが、不安定な場合があります。いつでもリリース版に戻せます。",
       "releaseTitle": "リリースチャンネルに切り替え",
-      "releaseMessage": "リリースに切り替えると、Gitリポジトリが削除され、最新の安定版がインストールされます。以降の更新は安定版のみが使用されます。",
+      "releaseMessage": "リリースに切り替えると、最新の安定版タグにチェックアウトされます。Gitリポジトリと設定は保持されます。いつでもNightlyに戻せます。",
       "switching": "{channel} チャンネルに切り替え中...",
       "completed": "{channel} チャンネルに切り替えました",
       "failed": "チャンネルの切り替えに失敗しました"

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"

locales/zh-TW.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": "切換到穩定版將檢出最新的發布標籤。Git 倉庫和您的設定均會保留。可隨時切換回每日構建版。",
       "switching": "正在切換到 {channel} 頻道...",
       "completed": "已切換到 {channel} 頻道",
       "failed": "切換頻道失敗"