fix(update): persist update_channel via settings.json instead of hasGit

After b464fdc3 (preserve .git on release switch), the hasGit-based
channel detection is unreliable — .git now exists for both release
and nightly installs, so page refresh always reset the channel.

- Add _resolveChannelFromSettings() with migration heuristic:
  !hasGit → release (ZIP), detached HEAD → release (on tag),
  on branch → nightly. Uses gitInfo.branch from check-updates.
- Persist resolved channel to settings.json on first load
  (one-time migration) and on explicit switchChannel.
- Add update_channel validation (release|nightly) in backend
  update_settings handler.
- Remove hasGit-based guessing from initialize(); defer to
  checkForUpdates where full gitInfo is available.
- Channel resolution runs before checkForUpdates early-returns
  to avoid null channelMode on reload-within-interval.

Tests: 361 passed.

py/routes/handlers/misc_handlers.py

@@ -1562,6 +1562,11 @@ class SettingsHandler:
                             {"success": False, "error": validation_error}
                         )
 
+                if key == "update_channel" and value not in ("release", "nightly"):
+                    return web.json_response(
+                        {"success": False, "error": "update_channel must be 'release' or 'nightly'"}
+                    )
+
                 if value == "__DELETE__" and key in (
                     "proxy_username",
                     "proxy_password",