Fix 2D control panel positioning relative to canvas

- Fix panel positioning calculation to account for panel height
- Position panel bottom edge 10% from canvas bottom (not container)
- Ensure panel stays within canvas bounds when WebGL canvas is active
- Update positioning on window resize for responsive behavior
- Resolves issue where panel appeared partially below canvas

index.html

@@ -31,7 +31,7 @@
 				<source src="sbs-video.mp4" type="video/mp4">
 			</video>
 			<button id="playBtn" aria-label="Play video">
-				<img src="play.svg" alt="Play">
+				<img src="play.png" alt="Play">
 			</button>
 			<div id="panel">
 				<div id="status">