Donate stranka: PAY by square QR s vyberom sumy

Linka Na kavu v hlavicke lobby vedie na /donate. QR sa generuje
dynamicky (balicek bysquare) pre presety 1/2/5 EUR alebo vlastnu sumu;
IBAN sa da skopirovat do schranky. Navstevy loguje existujuci
pageview beacon.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/pages/GameList.tsx

@@ -38,6 +38,9 @@ export default function GameList() {
           <button onClick={() => navigate('/history')} className="text-gold hover:text-gold-bright">
             História
           </button>
+          <button onClick={() => navigate('/donate')} className="text-gold hover:text-gold-bright">
+            Na kávu
+          </button>
           <button onClick={handleLogout} className="text-green-dim hover:text-gold">
             Odhlásiť
           </button>