Add admin role with an admin-only hand range study page

- Optional ADMIN_KEY (6 digits, distinct from ACCESS_KEY): entering it on the
  key screen signs in as admin (role-specific HMAC cookie; member cookies
  stay valid). Logged-in members can open /key?admin to enter it.
- /range (range chart + color/category quiz, ported from poker-hand and
  restyled to the site design) and its files under admin/ are served only to
  admins; everyone else gets 404. Card images cached privately for a day.
- Lobby shows a link to the page for admins only (GET /api/me).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

public/assets/lobby.js

@@ -22,6 +22,11 @@
     return false;
   }
 
+  // 管理者だけにハンドレンジ学習へのリンクを出す（ページ自体もサーバ側で管理者以外は 404）
+  App.api('GET', '/api/me')
+    .then((me) => { document.getElementById('range-link').hidden = !me.admin; })
+    .catch(() => {});
+
   document.getElementById('create').addEventListener('click', () => {
     if (requireName()) location.href = '/create';
   });