fix: add useRouter import to login and signup pages for navigation handling

feat: add user authentication checks and improve navigation with logout functionality

Fix all auth edge cases: proxy blocks+redirects properly, auth pages redirect if logged in, loading state prevents form flash, callback honors ?next param

Fix auth redirect URLs: use NEXT_PUBLIC_SITE_URL instead of window.location.origin (fixes localhost in verification emails)

v2: Fix all versions + minimal UI redesign (no gradients, clean layout)

🛡️ ClauseGuard v1.0 — Full codebase: Extension + Website + API + ML