feat: Convert about page from Jinja2 template to React component

[Marvinrose]

As part of the frontend revamp on the swatantrya branch, the about.html Jinja2 template needs to be converted to a React component to match the new frontend architecture.

This involves:

• Adding a /json/about API endpoint to expose the about page content
• Creating a new about.jsx route in frontend/src/routes/
• Registering the /about route in main.jsx
• Matching the existing component style and theming

[gridhead]

Issue closed via #881