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/item/definition.ts

@@ -320,7 +320,7 @@ export const itemDefinition: NodeDefinition<typeof ItemNode> = {
   presentation: {
     label: 'Item',
     description: 'A catalog-backed item (furniture, fixtures, decorations).',
-    icon: { kind: 'url', src: '/icons/item.png' },
+    icon: { kind: 'url', src: '/icons/item.webp' },
     paletteSection: 'furnish',
     paletteOrder: 10,
   },

packages/nodes/src/item/panel.tsx

@@ -100,7 +100,7 @@ export default function ItemPanel() {
 
   return (
     <PanelWrapper
-      icon={node.asset.thumbnail || '/icons/furniture.png'}
+      icon={node.asset.thumbnail || '/icons/furniture.webp'}
       onClose={handleClose}
       title={node.name || node.asset.name}
       width={300}