feat(web): wire auth pages with BetterAuth and route guards (#83)

Co-authored-by: Jason Woltje <jason@diversecanvas.com>
Co-committed-by: Jason Woltje <jason@diversecanvas.com>

apps/web/src/components/guest-guard.tsx

@@ -0,0 +1,35 @@
+'use client';
+
+import { useRouter } from 'next/navigation';
+import { useEffect } from 'react';
+import { useSession } from '@/lib/auth-client';
+
+interface GuestGuardProps {
+  children: React.ReactNode;
+}
+
+/** Redirects authenticated users away from auth pages. */
+export function GuestGuard({ children }: GuestGuardProps): React.ReactElement | null {
+  const { data: session, isPending } = useSession();
+  const router = useRouter();
+
+  useEffect(() => {
+    if (!isPending && session) {
+      router.replace('/chat');
+    }
+  }, [isPending, session, router]);
+
+  if (isPending) {
+    return (
+      <div className="flex min-h-screen items-center justify-center">
+        <div className="text-sm text-text-muted">Loading...</div>
+      </div>
+    );
+  }
+
+  if (session) {
+    return null;
+  }
+
+  return <>{children}</>;
+}