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/duct-fitting/definition.ts

@@ -122,7 +122,7 @@ export const ductFittingDefinition: NodeDefinition<typeof DuctFittingNode> = {
   presentation: {
     label: 'Duct Fitting',
     description: 'Elbow, tee, reducer, or square-to-round transition connecting duct runs.',
-    icon: { kind: 'url', src: '/icons/duct-fitting.png' },
+    icon: { kind: 'url', src: '/icons/duct-fitting.webp' },
     paletteSection: 'structure',
     paletteOrder: 91,
   },