Update environment configuration and routing for OAuth and frontend integration

Commit notes

- Adjusted allowed origins in env.example and config.go for consistent CORS handling. - Updated Google OAuth redirect URL to reflect new API versioning in both env.example and config.go. - Changed default frontend URL from localhost:5173 to localhost:3000 in multiple files for uniformity. - Added new route for "/auth/callback" in frontend routing and created corresponding type definitions. - Refactored Header component to include AccountPopover for enhanced user experience.