perf: 100% Lighthouse score — contrast, CLS, caching fixes

- Font preload eliminates layout shift (CLS 0)
- Dual accent colors: --accent for filled buttons, --accent-text for text on dark bg
- All WCAG AA contrast ratios met (buttons, links, badges, countdown)
- Language picker: position absolute instead of fixed
- CoinGecko rates proxied with 2min server-side cache (no CORS, no rate limit)
- English as default inline text (no empty→text shift)

api/rates.php

@@ -1,9 +1,21 @@
 <?php
 header('Content-Type: application/json');
-header('Cache-Control: public, max-age=60');
+
+// Server-side cache: fetch from CoinGecko at most once per 2 minutes
+$cacheFile = __DIR__ . '/../data/rates_cache.json';
+$cacheTTL = 120; // seconds
+
+if (file_exists($cacheFile)) {
+    $cached = json_decode(file_get_contents($cacheFile), true);
+    if ($cached && (time() - ($cached['_time'] ?? 0)) < $cacheTTL) {
+        unset($cached['_time']);
+        header('Cache-Control: public, max-age=60');
+        echo json_encode($cached);
+        exit;
+    }
+}
 
 $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,
@@ -15,8 +27,27 @@ $httpCode = curl_getinfo($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']);
+    $data = json_decode($response, true);
+    if ($data) {
+        $data['_time'] = time();
+        file_put_contents($cacheFile, json_encode($data));
+        unset($data['_time']);
+        header('Cache-Control: public, max-age=60');
+        echo json_encode($data);
+        exit;
+    }
 }
+
+// On error, serve stale cache if available
+if (file_exists($cacheFile)) {
+    $cached = json_decode(file_get_contents($cacheFile), true);
+    if ($cached) {
+        unset($cached['_time']);
+        header('Cache-Control: public, max-age=30');
+        echo json_encode($cached);
+        exit;
+    }
+}
+
+http_response_code(502);
+echo json_encode(['error' => 'Failed to fetch rates']);