0.5.0

feat: silent background updates with checking toast and opt-out

Updates now behave like Discord/VS Code: when a check finds a new
version it downloads silently in the background and only surfaces a
'Restart to update' prompt once ready (still applied on quit via
autoInstallOnAppQuit if never clicked).

- New bottom-right toast: 'Checking for updates…' on startup, then
  'You're up to date' (auto-fades) or update found/ready states
- Home banner hides during silent downloads; sidebar shows passive
  progress; all surfaces switch to Restart to update when ready
- New 'Download updates automatically' setting (default on); turning
  it off restores the manual Download buttons everywhere, and turning
  it back on mid-session fetches any pending update immediately
- Re-checks no longer re-toast the same version every 5 minutes
- Bump version to 0.5.0

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>