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

style.css

@@ -320,8 +320,8 @@ textarea {
 }
 
 .btn-primary:disabled {
-  background: #5a3520;
-  color: #bbb;
+  background: #3a2215;
+  color: #a0a0a0;
   cursor: not-allowed;
 }