feat(m3): auth & user management with session, tests, and e2e

- Custom error classes (lib/errors.ts) and signed-cookie session
  (lib/auth/session.ts, HMAC-SHA256, httpOnly+sameSite=lax)
- UserRepository + AuthService (bcrypt hashing, role/status checks,
  register/login/logout/getCurrentUser/updateProfile)
- Auth helpers (getCurrentUser), user validator, API error handler
- API routes: register (auto-login), login, logout, me, PATCH users/me,
  GET admin/migrations (admin-only guard)
- middleware.ts redirects unauthenticated page access to /login
- Screens: login (login/register toggle), profile, dashboard skeleton,
  home redirect
- vitest: exclude e2e specs, setupFiles for SESSION_SECRET
- playwright: run migrate before dev server, set SESSION_SECRET
- Unit tests (UserRepository, AuthService, session), integration
  auth-flow, e2e auth.spec (register/login/logout/protected/401)

app/page.tsx

@@ -1,10 +1,12 @@
+'use client';
+
+import { useEffect } from 'react';
+import { useRouter } from 'next/navigation';
+
 export default function Home() {
-  return (
-    <main className="flex min-h-screen flex-col items-center justify-center p-8">
-      <h1 className="text-3xl font-bold">シンプルグループウェア</h1>
-      <p className="mt-4 text-gray-600">
-        プロジェクト単位で情報共有・タスク管理を行えるチームコラボレーションツール
-      </p>
-    </main>
-  );
+  const router = useRouter();
+  useEffect(() => {
+    router.replace('/dashboard');
+  }, [router]);
+  return null;
 }