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/package.json

@@ -0,0 +1,36 @@
+{
+  "name": "client",
+  "private": true,
+  "version": "0.0.0",
+  "type": "module",
+  "scripts": {
+    "dev": "vite",
+    "build": "tsc -b && vite build",
+    "preview": "vite preview"
+  },
+  "dependencies": {
+    "react": "^19.0.0",
+    "react-dom": "^19.0.0",
+    "react-router-dom": "^7.1.0",
+    "class-variance-authority": "^0.7.1",
+    "clsx": "^2.1.1",
+    "tailwind-merge": "^2.6.0",
+    "lucide-react": "^0.469.0",
+    "@radix-ui/react-slot": "^1.1.1",
+    "@radix-ui/react-separator": "^1.1.1",
+    "@radix-ui/react-dialog": "^1.1.4",
+    "@radix-ui/react-label": "^2.1.1",
+    "@radix-ui/react-select": "^2.1.4",
+    "@radix-ui/react-tabs": "^1.1.2",
+    "@radix-ui/react-toast": "^1.2.4"
+  },
+  "devDependencies": {
+    "@types/react": "^19.0.0",
+    "@types/react-dom": "^19.0.0",
+    "@vitejs/plugin-react": "^4.3.4",
+    "typescript": "~5.7.0",
+    "vite": "^6.0.0",
+    "tailwindcss": "^4.0.0",
+    "@tailwindcss/vite": "^4.0.0"
+  }
+}