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/templates/index.html

@@ -152,6 +152,10 @@
         </footer>
     </div>
 
+    <button id="scroll-to-top" class="scroll-to-top" onclick="window.scrollTo({top:0,behavior:'smooth'})" aria-label="Scroll to top">
+        <svg class="icon" viewBox="0 0 24 24"><path d="m18 15-6-6-6 6"/></svg>
+    </button>
+
     <div id="toast" class="toast"></div>
 
     {% include 'modals/calibration.html' %}