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/elevator/definition.ts

@@ -256,7 +256,7 @@ export const elevatorDefinition: NodeDefinition<typeof ElevatorNode> = {
   presentation: {
     label: 'Elevator',
     description: 'A multi-level elevator shaft with configurable openings per level.',
-    icon: { kind: 'url', src: '/icons/wallcut.png' },
+    icon: { kind: 'url', src: '/icons/wallcut.webp' },
     paletteSection: 'structure',
     paletteOrder: 80,
   },

packages/nodes/src/elevator/panel.tsx

@@ -471,7 +471,7 @@ export default function ElevatorPanel() {
 
   return (
     <PanelWrapper
-      icon="/icons/elevator.png"
+      icon="/icons/elevator.webp"
       onClose={handleClose}
       title={node.name || 'Elevator'}
       width={300}