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/dashboard/page.tsx

@@ -0,0 +1,27 @@
+import { redirect } from 'next/navigation';
+import { getCurrentUser } from '@/lib/auth/getCurrentUser';
+
+export const dynamic = 'force-dynamic';
+
+export default async function DashboardPage() {
+  const user = await getCurrentUser();
+  if (!user) {
+    redirect('/login');
+  }
+
+  return (
+    <main className="min-h-screen bg-gray-50 p-8">
+      <div className="mx-auto max-w-3xl">
+        <div className="flex items-center justify-between">
+          <h1 className="text-2xl font-bold">ダッシュボード</h1>
+          <a href="/profile" className="text-sm text-blue-600 hover:underline">
+            {user.name} さん
+          </a>
+        </div>
+        <p className="mt-4 text-gray-600">
+          プロジェクト機能は後続マイルストーンで実装されます。
+        </p>
+      </div>
+    </main>
+  );
+}