Add scroll-to-top button

Fixed button in bottom-right corner appears after scrolling 300px,
fades in with slide-up animation, smooth-scrolls to page top on click.

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

server/src/wled_controller/static/js/app.js

@@ -536,6 +536,14 @@ document.addEventListener('DOMContentLoaded', async () => {
         window.addEventListener('resize', updateHeaderHeight);
     }
 
+    // Scroll-to-top button visibility
+    const scrollBtn = document.getElementById('scroll-to-top');
+    if (scrollBtn) {
+        window.addEventListener('scroll', () => {
+            scrollBtn.classList.toggle('visible', window.scrollY > 300);
+        }, { passive: true });
+    }
+
     // Initialize command palette
     initCommandPalette();