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>

apps/editor/components/scene-loader.tsx

@@ -42,7 +42,7 @@ const SIDEBAR_TABS: (SidebarTab & { component: React.ComponentType })[] = [
         alt=""
         className="h-8 w-8 object-contain"
         height={32}
-        src="/icons/scene.png"
+        src="/icons/scene.webp"
         width={32}
       />
     ),
@@ -58,7 +58,7 @@ const SIDEBAR_TABS: (SidebarTab & { component: React.ComponentType })[] = [
         alt=""
         className="h-8 w-8 object-contain"
         height={32}
-        src="/icons/build.png"
+        src="/icons/build.webp"
         width={32}
       />
     ),