fix: Ensure controls panel and buttons are properly visible

Add explicit positioning and z-index to controls panel and buttons
to ensure they render correctly and are clickable in all browsers,
especially during E2E testing.

- Add z-index: 10 to controls-panel
- Add pointer-events: auto to buttons
- Add relative positioning to ensure layering works correctly

Author: claude

examples/webgpu-cube/index.html

@@ -96,6 +96,8 @@
             background: rgba(255, 255, 255, 0.03);
             border-radius: 12px;
             border: 1px solid rgba(255, 255, 255, 0.1);
+            position: relative;
+            z-index: 10;
         }
 
         /* Arrow Buttons Layout */
@@ -128,6 +130,9 @@
             align-items: center;
             justify-content: center;
             user-select: none;
+            pointer-events: auto;
+            position: relative;
+            z-index: 1;
         }
 
         .control-button:hover {