feat(web): lighten tabletop scene and tune contact shadows

Replace the dark surface plane with a grey background and a spotlight,
size the contact shadow to the table, and scale its blur and fade with
the shadow plane. Disable depth test on the stacking curve overlay.

packages/tabletop/src/index.ts

@@ -8,6 +8,7 @@ export {
   fallbackShape,
   traceToShape,
   traceToUvBounds,
+  MM_TO_WORLD,
 } from './part.js';
 export { resolveAssetUrl, assetUrl, traceImage } from './http.js';
 
@@ -34,7 +35,6 @@ export type { MountNode, MountTree } from './mount.js';
 export { WorldSurfaceView, SurfaceNode } from './surfaces/WorldSurfaceView.js';
 export { HudSurfaceView } from './surfaces/HudSurfaceView.js';
 export { SurfaceBounds } from './surfaces/SurfaceBounds.js';
-export { SurfacePlane } from './surfaces/SurfacePlane.js';
 
 // Part placement.
 export { PartPlacement } from './placement.js';