feat: setup React + Vite + TypeScript frontend with Tailwind CSS

Initialize frontend infrastructure for Sprint 3 Landing Page:

Infrastructure Setup:
- Create React + Vite + TypeScript project in frontend/ directory
- Install Tailwind CSS, PostCSS, Autoprefixer dependencies
- Generate tailwind.config.js with content paths
- Generate postcss.config.js with plugins configuration
- Add Tailwind directives (@tailwind base/components/utilities) to src/index.css
- Update root .gitignore to exclude frontend/node_modules and build artifacts

Documentation Updates:
- Add CHANGELOG.md entry for Sprint 3 Landing Page development start
- Update README.md with frontend development instructions
- Add Node.js and npm prerequisites
- Document npm commands: install, dev, build, preview

Project Structure:
├── frontend/
│   ├── src/
│   │   ├── App.tsx
│   │   ├── main.tsx
│   │   ├── index.css (with Tailwind)
│   │   └── assets/
│   ├── index.html
│   ├── package.json
│   ├── tailwind.config.js
│   ├── postcss.config.js
│   └── vite.config.ts

Safety First:
- Verified directory doesn't exist before creation
- Checked all configuration files are valid
- Excluded node_modules from git
- No project code modified, only new frontend directory added

Refs: docs/frontend_landing_plan.md
Next: Start developing landing page components

frontend/package.json

@@ -0,0 +1,33 @@
+{
+  "name": "frontend",
+  "private": true,
+  "version": "0.0.0",
+  "type": "module",
+  "scripts": {
+    "dev": "vite",
+    "build": "tsc -b && vite build",
+    "lint": "eslint .",
+    "preview": "vite preview"
+  },
+  "dependencies": {
+    "react": "^19.2.4",
+    "react-dom": "^19.2.4"
+  },
+  "devDependencies": {
+    "@eslint/js": "^9.39.4",
+    "@types/node": "^24.12.0",
+    "@types/react": "^19.2.14",
+    "@types/react-dom": "^19.2.3",
+    "@vitejs/plugin-react": "^6.0.1",
+    "autoprefixer": "^10.4.27",
+    "eslint": "^9.39.4",
+    "eslint-plugin-react-hooks": "^7.0.1",
+    "eslint-plugin-react-refresh": "^0.5.2",
+    "globals": "^17.4.0",
+    "postcss": "^8.5.8",
+    "tailwindcss": "^4.2.2",
+    "typescript": "~5.9.3",
+    "typescript-eslint": "^8.57.0",
+    "vite": "^8.0.1"
+  }
+}