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.
