Add tray "Check for Updates" item and launch notification

Wire a "Check for Updates" entry into the tray menu that runs a one-shot
rsupd self-update off the tray thread, reusing the same signed updater
(build_updater) as the hourly background task. On launch, pop a desktop
notification pointing the user at the tray icon.

Adds notify-rust (zbus backend on Linux, WinRT toast on Windows), gated
behind the gui feature.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: MagicalTux