feat(update): add Release/Nightly channel switching

- Add POST /api/lm/switch-channel endpoint with git init / ZIP fallback
- Add _backup_git/_restore_git helpers with safe rollback
- Version-info endpoint now returns has_git flag for auto-detection
- Check-updates always returns releases (changelog) regardless of channel
- Nightly mode shows 'N commits behind main' with commit hash and date
- View on GitHub link points to /commits/main in nightly mode
- Channel toggle UI with pill-style buttons in update modal
- Confirmation dialog with Esc / backdrop-dismiss support
- Channel derived from has_git on every page load, no localStorage
- i18n: 11 new keys translated across 9 non-English locales
- CSS: unified card-style sections in _base.css
- Tests: 8 new tests covering switch-channel, nightly response, init_git_repo

locales/zh-CN.json

@@ -1752,6 +1752,12 @@
     "checkingMessage": "请稍候，正在检查最新版本。",
     "showNotifications": "显示更新通知",
     "latestBadge": "最新",
+    "latestMain": "Main 分支",
+    "channel": "更新频道",
+    "channels": {
+      "release": "稳定版",
+      "nightly": "Nightly"
+    },
     "updateProgress": {
       "preparing": "正在准备更新...",
       "installing": "正在安装更新...",
@@ -1772,6 +1778,15 @@
       "warning": "警告：Nightly 版本可能包含实验性功能，可能不稳定。",
       "enable": "启用 Nightly 更新"
     },
+    "channelSwitch": {
+      "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。",
+      "switching": "正在切换到 {channel} 频道...",
+      "completed": "已切换到 {channel} 频道",
+      "failed": "切换频道失败"
+    },
     "banners": {
       "recent": "最近的通知",
       "empty": "暂无最近的横幅通知。",
@@ -2235,4 +2250,4 @@
       "retry": "重试"
     }
   }
-}
+}