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/cupola/definition.ts

@@ -153,7 +153,7 @@ export const cupolaDefinition: NodeDefinition<typeof CupolaNode> = {
   presentation: {
     label: 'Cupola',
     description: 'Louvered roof lantern with a dome or pyramid cap and optional finial.',
-    icon: { kind: 'url', src: '/icons/roof.png' },
+    icon: { kind: 'url', src: '/icons/roof.webp' },
     paletteSection: 'structure',
     paletteOrder: 122,
   },

packages/nodes/src/cupola/panel.tsx

@@ -137,7 +137,7 @@ export default function CupolaPanel() {
 
   return (
     <PanelWrapper
-      icon="/icons/roof.png"
+      icon="/icons/roof.webp"
       onBack={node.roofSegmentId ? handleBack : undefined}
       onClose={handleClose}
       title={node.name || 'Cupola'}