Desktop Google login flow: click "Continue with Google" → opens default
browser to web login page with platform=desktop → Google OAuth completes
→ web callback redirects to multica://auth/callback?token=<jwt> →
Electron receives deep link, extracts token, completes login.
Changes:
- Register `multica://` protocol in Electron (main process + builder)
- Add single-instance lock with deep link forwarding (macOS + Win/Linux)
- Expose `desktopAPI.onAuthToken` and `openExternal` via preload IPC
- Add `loginWithToken(token)` to core auth store
- Pass `state=platform:desktop` through Google OAuth flow
- Web callback detects desktop state and redirects via deep link
- Desktop renderer listens for auth token and hydrates session
Check for updates on startup via electron-updater. When a new version is
detected, show a notification in the bottom-right corner with download
and restart-to-install actions.
