Adds a polite, dismissible banner between the header and navigation
tabs that appears when the local repo is behind origin/main. Shows
commit count and a one-click "Update Now" button that triggers the
existing git_pull action.
- New GET /api/v3/system/check-update endpoint (5-min cache, compares
local HEAD vs origin/main SHA)
- Banner auto-checks on page load then every 30 minutes
- Dismiss persists for the browser session via sessionStorage
- Styled for both light and dark themes
- Cache invalidated after successful git_pull so banner hides immediately
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
