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

.gitignore

@@ -26,3 +26,10 @@ Thumbs.db
 
 # Telegram bot configuration (contains sensitive tokens)
 docs/telegram_setup.md
+
+# Frontend dependencies
+frontend/node_modules/
+frontend/dist/
+frontend/.env
+frontend/.env.local
+frontend/.env.*.local