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)

lib/auth/constants.ts

@@ -0,0 +1,6 @@
+/**
+ * セッションCookie関連の定数。
+ * Edge Runtime(middleware)からも安全に参照できるよう、依存を持たない定数のみ配置する。
+ */
+export const SESSION_COOKIE = 'session';
+export const SESSION_MAX_AGE_SECONDS = 7 * 24 * 60 * 60;