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

api/rates.php

@@ -0,0 +1,22 @@
+<?php
+header('Content-Type: application/json');
+header('Cache-Control: public, max-age=60');
+
+$url = 'https://api.coingecko.com/api/v3/simple/price?ids=monero&vs_currencies=eur,usd,chf';
+
+$ch = curl_init($url);
+curl_setopt_array($ch, [
+    CURLOPT_RETURNTRANSFER => true,
+    CURLOPT_TIMEOUT => 10,
+    CURLOPT_HTTPHEADER => ['Accept: application/json'],
+]);
+$response = curl_exec($ch);
+$httpCode = curl_getinfo($ch, CURLINFO_HTTP_CODE);
+curl_close($ch);
+
+if ($response !== false && $httpCode === 200) {
+    echo $response;
+} else {
+    http_response_code(502);
+    echo json_encode(['error' => 'Failed to fetch rates']);
+}