Update Settings Fluent Controls

Goal

Make the Settings > Update page the single user-facing control surface for the host update flow.

The page uses Fluent Avalonia settings controls for update status, release facts, update behavior, and transfer controls.
Users can choose update channel, download source, update mode, and download thread count.
Update mode options are:
- Manual: do not automatically download or install.
- Silent Download: check and download in the background, then wait for user installation confirmation.
- Silent Install: check and download in the background, then apply when the app exits.
Users can opt into forced reinstall. When enabled, the update check targets the current version manifest where available and the UI labels the next payload as reinstall.
The page displays whether the current payload is an incremental update or reinstall/full installer.
The page exposes pause, resume, and cancel actions for resumable downloads and install recovery.
Existing PloNDS/FileMap incremental update behavior remains, but update apply and rollback ownership belongs to the Host. Launcher only selects and starts the current app version.
The page follows ClassIsland's durable-status vs working-status split: a transient check/download error must not be treated as an available update, and available/downloaded actions must stay visible while the worker is idle.

UpdateSettingsPage shows Fluent Avalonia controls for channel, mode, thread count, forced reinstall, pause/resume, and cancel.
UpdateSettingsState persists forced reinstall alongside other update preferences.
Automatic startup checks skip manual mode, download in silent download/silent install modes, and leave installation to explicit user action or exit-time apply.
After a successful check with an available update, the download action is visible even though no transfer is running.
After a failed check, no download action is shown unless a valid update is still pending.
Build succeeds for LanMountainDesktop.slnx.