* feat: add update reminder component and background polling
* refactor: replace update reminder banner with subtle bottom-right toast
Swap the full-width top banner for a compact fixed-position toast in the
bottom-right corner, matching the existing Sonner toast styling. Also
extract pure functions (statusesEqual, isBenignCheckFailure) from
updater.ts into updater-fallback.ts to fix max-lines lint error.
* fix: mock checkForUpdates to return a promise so .catch() works in tests