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>

package.json

@@ -40,7 +40,9 @@
     "format": "prettier --write 'src/**/*.{ts,tsx}'",
     "format:check": "prettier --check 'src/**/*.{ts,tsx}'",
     "test": "vitest run --passWithNoTests",
-    "test:watch": "vitest"
+    "test:watch": "vitest",
+    "demo:dev": "vite -c vite.demo.config.ts --mode arrangement",
+    "demo:build": "vite build -c vite.demo.config.ts --mode arrangement"
   },
   "dependencies": {
     "@emotion/react": "^11.13.0",