Fix sticky header on mobile by using overflow-x: clip instead of hidden

overflow-x: hidden on body creates a scroll container that breaks
position: sticky. overflow-x: clip prevents horizontal overflow
without affecting the sticky context.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/static/css/mobile.css

@@ -92,9 +92,9 @@
    PHONE  (≤ 600px)
    ================================================================ */
 @media (max-width: 600px) {
-    /* Prevent horizontal scroll */
+    /* Prevent horizontal scroll (clip instead of hidden to preserve sticky) */
     html, body {
-        overflow-x: hidden;
+        overflow-x: clip;
     }
 
     /* ── Header ── */