perf(editor): convert UI icons from PNG to WebP

The editor's /icons assets were ~10MB of oversized PNGs (a single
toolbar icon up to 1.4MB). Convert every non-PWA icon to WebP (quality
92) and repoint all /icons/*.png references to .webp across
packages/editor, packages/nodes, and apps/editor. PWA/platform icons
(apple-touch-icon, icon-192, icon-512) stay PNG.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/nodes/src/slab/definition.ts

@@ -208,7 +208,7 @@ export const slabDefinition: NodeDefinition<typeof SlabNode> = {
   presentation: {
     label: 'Slab',
     description: 'A polygon-bounded floor surface that hosts items on top.',
-    icon: { kind: 'url', src: '/icons/floor.png' },
+    icon: { kind: 'url', src: '/icons/floor.webp' },
     paletteSection: 'structure',
     paletteOrder: 30,
   },

packages/nodes/src/slab/panel.tsx

@@ -152,7 +152,7 @@ export function SlabPanel() {
 
   return (
     <PanelWrapper
-      icon="/icons/floor.png"
+      icon="/icons/floor.webp"
       onClose={handleClose}
       title={node.name || 'Slab'}
       width={320}