perf: CoinGecko proxy, font-display optional, contrast fix

- Route CoinGecko API through /api/rates.php to avoid CORS blocks
- font-display: optional eliminates font-swap layout shifts (CLS ~0)
- Disabled button contrast: #bbb on #5a3520 (5.8:1 ratio)
- Nginx font caching: 1 year, immutable

app.js

@@ -2,7 +2,7 @@
   'use strict';
 
   // --- Config ---
-  const COINGECKO_API = 'https://api.coingecko.com/api/v3/simple/price?ids=monero&vs_currencies=eur,usd,chf';
+  const COINGECKO_API = '/api/rates.php';
   // Standard address (4..., 95 chars), Subaddress (8..., 95 chars), Integrated address (4..., 106 chars)
   const XMR_STANDARD_REGEX = /^[48][1-9A-HJ-NP-Za-km-z]{94}$/;
   const XMR_INTEGRATED_REGEX = /^4[1-9A-HJ-NP-Za-km-z]{105}$/;