Replace vanilla HTML with React + shadcn/ui + TailwindCSS scaffold

- Frontend: React 19 + Vite + TypeScript + TailwindCSS v4 + shadcn/ui
- Pre-installed shadcn/ui components: Button, Card, Input, Separator
- Multi-stage Dockerfile: build React client → serve with Express
- Server.js updated to serve built React SPA with API routes
- Sample landing page with shadcn/ui components demonstrating usage
- Path aliases (@/) configured in tsconfig + vite config
- components.json for easy `shadcn add <component>` usage

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

client/src/App.tsx

@@ -0,0 +1,14 @@
+import { BrowserRouter, Routes, Route } from 'react-router-dom'
+import HomePage from '@/pages/Home'
+
+function App() {
+  return (
+    <BrowserRouter>
+      <Routes>
+        <Route path="/" element={<HomePage />} />
+      </Routes>
+    </BrowserRouter>
+  )
+}
+
+export default App