dx: Improve GitHub OAuth redirect flow

[Nishchit14]

Description

The current GitHub OAuth redirect URL used after login is https://firecamp.dev/identity.html. This requires having a special identity.html page just for handling the redirect.

Instead, we could simplify the flow by redirecting directly to the main / route after login. This would allow removing the identity.html page and associated handler code.

Steps to implement:

• Update GitHub OAuth config to set redirect URL to https://firecamp.dev/ rather than the identity page
• Remove the identity.html page and routing code that points to it
• Update main / route handler to check for authorization code in URL query params
• If auth code is present, exchange it for a token and store it before redirecting to the actual / route
• Modify other parts of the app that link to identity.html to point to / instead

This would clean up the auth flow and reduce redundant code.