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/guide/definition.ts

@@ -45,7 +45,7 @@ export const guideDefinition: NodeDefinition<typeof GuideNode> = {
   presentation: {
     label: 'Guide',
     description: 'A measurement / reference annotation (linear, area, or arc).',
-    icon: { kind: 'url', src: '/icons/blueprint.png' },
+    icon: { kind: 'url', src: '/icons/blueprint.webp' },
     paletteSection: 'site',
     paletteOrder: 30,
   },