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>

vite.demo.config.ts

@@ -0,0 +1,41 @@
+import { defineConfig } from 'vite';
+import react from '@vitejs/plugin-react';
+import path from 'path';
+
+/**
+ * Per-slice demo build. Slice name comes from `--mode <name>` and selects
+ * the app folder, base path, and output directory.
+ *
+ * Dev:    vite -c vite.demo.config.ts --mode arrangement
+ * Build:  vite build -c vite.demo.config.ts --mode arrangement
+ *         → dist-demo/arrangement/
+ */
+export default defineConfig(({ mode, command }) => {
+  const slice = mode;
+  const appRoot = path.resolve(__dirname, `src/demo/apps/${slice}`);
+
+  return {
+    root: appRoot,
+    envDir: __dirname,
+    base: command === 'build' ? `/${slice}/` : '/',
+    publicDir: path.resolve(__dirname, 'brandassets'),
+    plugins: [react()],
+    resolve: {
+      alias: {
+        '@atoms': path.resolve(__dirname, 'src/components/atoms'),
+        '@molecules': path.resolve(__dirname, 'src/components/molecules'),
+        '@organisms': path.resolve(__dirname, 'src/components/organisms'),
+        '@theme': path.resolve(__dirname, 'src/theme'),
+      },
+    },
+    build: {
+      outDir: path.resolve(__dirname, `dist-demo/${slice}`),
+      emptyOutDir: true,
+      sourcemap: true,
+    },
+    server: {
+      port: 5180,
+      open: false,
+    },
+  };
+});