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

i18n.js

@@ -115,7 +115,7 @@ var I18n = (function () {
       var code = navLangs[i].substring(0, 2).toLowerCase();
       if (translations[code]) return code;
     }
-    return 'de';
+    return 'en';
   }
 
   function applyDOM(t) {