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/pipe-fitting/definition.ts

@@ -93,7 +93,7 @@ export const pipeFittingDefinition: NodeDefinition<typeof PipeFittingNode> = {
     label: 'Pipe Fitting',
     description: 'DWV joint — elbow bend, 45° wye, or sanitary tee.',
     // Reuses the duct-fitting artwork — DWV fittings read the same in the UI.
-    icon: { kind: 'url', src: '/icons/duct-fitting.png' },
+    icon: { kind: 'url', src: '/icons/duct-fitting.webp' },
     paletteSection: 'structure',
     paletteOrder: 96,
     hidden: true,