perf: eliminate CLS with inline default text, EN as default language

- All visible text pre-rendered in HTML (no empty→text shift)
- English as default language (i18n fallback + HTML inline text)
- German auto-detected via navigator.languages for DE browsers
- font-display: optional (no font-swap shift)
- Disabled button contrast fix (#a0a0a0 on #3a2215, 7.2:1)
- CoinGecko rates proxied via /api/rates.php with User-Agent

api/rates.php

@@ -8,7 +8,7 @@ $ch = curl_init($url);
 curl_setopt_array($ch, [
     CURLOPT_RETURNTRANSFER => true,
     CURLOPT_TIMEOUT => 10,
-    CURLOPT_HTTPHEADER => ['Accept: application/json'],
+    CURLOPT_HTTPHEADER => ['Accept: application/json', 'User-Agent: xmrpay.link/1.0'],
 ]);
 $response = curl_exec($ch);
 $httpCode = curl_getinfo($ch, CURLINFO_HTTP_CODE);