Replaces the custom-scheme auto-redirect (which triggers Chrome's
confirmation prompt) with a verified Android App Link handoff:
- public/.well-known/assetlinks.json for space.einundzwanzig.mobile
(debug cert fingerprint; add the release cert before store builds)
- GET /app/auth handoff: opens the app directly when the App Link is
verified; renders a button-based fallback page otherwise
- POST /api/mobile/token: trades a NIP-55-signed login event for a
Sanctum token — used when Amber's callback opens the app directly
- complete/confirm/signedCallback now redirect to the handoff URL
HolgerHatGarKeineNode