Add arrangement demo site from original project

Copies the multi-page demo app (Providers → Packages → Comparison flow)
with Zustand basket state, URL sync, and per-slice Vite build config.
All pages render correctly on React 19 + MUI v7 with zero code changes needed.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/demo/shared/assets.ts

@@ -0,0 +1,17 @@
+/**
+ * Resolve a public-asset path against Vite's base URL.
+ *
+ * In dev `import.meta.env.BASE_URL === '/'`, so `assetUrl('/images/foo.png')`
+ * returns `/images/foo.png` unchanged. In production the build sets base to
+ * `/arrangement/` (or whatever `--mode <slice>` was passed), and the same
+ * call returns `/arrangement/images/foo.png` so the bundled assets resolve
+ * correctly under the slice subpath.
+ *
+ * Always pass leading-slash paths — they're relative to the publicDir root.
+ */
+export const assetUrl = (path: string): string => {
+  const base = import.meta.env.BASE_URL;
+  const cleanBase = base.endsWith('/') ? base.slice(0, -1) : base;
+  const cleanPath = path.startsWith('/') ? path : `/${path}`;
+  return `${cleanBase}${cleanPath}`;
+};