fix: opaque background on nav dropdown to match page background

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: aymuos15

styles/components/header.css

@@ -54,7 +54,9 @@
     display: flex;
     flex-direction: column;
     gap: 0;
-    padding: 0;
+    padding: 0.4rem 0.6rem;
+    background: var(--bg);
+    border-radius: 4px;
     opacity: 0;
     pointer-events: none;
     transform: translateY(-8px);