Add PWA support for offline browsing of cached repos

[AditthyaSS]

The app is fully client-side with an existing IndexedDB cache layer — it's a natural candidate for a Progressive Web App:

Add manifest.json with app name, icons, theme color
Register a service worker for asset caching
Allow offline browsing of previously cached repos
Add "Install App" prompt for mobile users
This would significantly improve the mobile experience and allow users to browse cached data without an internet connection.

[AkhileshDasari]

Hi! I'd like to work on this issue as part of ECSoC. Could you please assign it to me?

[AkhileshDasari]

Closes #38
PWA implementation completed ✅

• Added manifest.json with proper app metadata, icons, and theme color
• Registered service worker for caching and offline support
• Implemented install prompt for mobile and desktop users
• Added friendly offline experience using Forky
• App is now installable as a standalone PWA

Tested with Lighthouse and offline mode. Users can now install Forkling and browse cached repositories without internet.

Ready for review! 🦎📱