Convert /drinkkaart route from a redirect to a full admin scanning interface.
Add flow: scan QR → resolve user → select amount → confirm deduction → success.
Refactor QrScanner to use Html5Qrcode API for better control and cleanup.
Update event date/time to April 24, 2026 at 19:30.
BREAKING CHANGE: /drinkkaart now requires admin role and provides scanner UI
instead of redirecting to /account.
