feat(hackathon): improve auth flow and UX

- Add email activation requirement for signup (no auto-login)
- Add form validation with React Hook Form and Zod on signup page
- Update callback page to handle email confirmation and password reset redirects
- Fix token format in API interceptor (use access_token)
- Fix middleware to use SessionToken for auth check
- Add infinite scroll with IntersectionObserver on browse teams page
- Replace all internal <a href> with <Link> components
- Add useInfiniteTeams hook for paginated team browsing

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

apps/hackathon/src/app/auth/login/page.tsx

@@ -172,12 +172,12 @@ export default function LoginPage() {
         <div className="mt-6 text-center">
           <p className="text-gray-600 dark:text-gray-400 text-sm">
             Don't have an account?{' '}
-            <a
-              href="/auth/signup"
+            <Link
+              to="/auth/signup"
               className="text-primary-600 dark:text-primary-400 hover:text-primary-700 dark:hover:text-primary-300 font-semibold"
             >
               Sign up
-            </a>
+            </Link>
           </p>
         </div>