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/lobby.html

@@ -18,6 +18,7 @@
     <div class="error-text" id="name-error"></div>
 
     <button id="create" class="btn btn-orange btn-block mt-4" type="button">＋ テーブルを作成</button>
+    <a id="range-link" class="btn btn-dark btn-block mt-8" href="/range" hidden>♠ ハンドレンジ学習</a>
 
     <div class="section-title">
       <span>開催中のテーブル</span>