feat(content): add About Us page with markdown content support

- Create AboutUsPage component with markdown rendering
- Add react-markdown library for content rendering
- Create public/content directory for editable markdown files
- Add about-us.md with Lorem Ipsum placeholder content
- Create public/images/about directory for page images
- Add /about-us route in App.jsx
- Add About Us link to homepage footer
- Support for external links (open in new tab) and internal links
- Responsive image rendering with rounded corners and shadow

frontend/package.json

@@ -19,6 +19,7 @@
     "react": "^19.2.0",
     "react-dom": "^19.2.0",
     "react-hot-toast": "^2.6.0",
+    "react-markdown": "^10.1.0",
     "react-router-dom": "^7.9.5",
     "socket.io-client": "^4.8.1"
   },