Add LoginLink client component that captures current path via usePathname()
and passes it as returnTo query param. Update login page to read and
sanitize returnTo (open-redirect guard: must start with /, reject //).
Update LoginForm to redirect to returnTo after successful login. Wire
LandingPage CTA through LoginLink. Add returnTo redirect for already-
authed users hitting /login. Add return-URL to flappy page redirect.
Remove trailing blank line from (frontend) layout.
