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/en.json

@@ -1752,6 +1752,12 @@
     "checkingMessage": "Please wait while we check for the latest version.",
     "showNotifications": "Show update notifications",
     "latestBadge": "Latest",
+    "latestMain": "Latest main",
+    "channel": "Update Channel",
+    "channels": {
+      "release": "Release",
+      "nightly": "Nightly"
+    },
     "updateProgress": {
       "preparing": "Preparing update...",
       "installing": "Installing update...",
@@ -1772,6 +1778,15 @@
       "warning": "Warning: Nightly builds may contain experimental features and could be unstable.",
       "enable": "Enable Nightly Updates"
     },
+    "channelSwitch": {
+      "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.",
+      "switching": "Switching to {channel} channel...",
+      "completed": "Successfully switched to {channel} channel",
+      "failed": "Failed to switch channel"
+    },
     "banners": {
       "recent": "Recent messages",
       "empty": "No recent banners yet.",