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>

server/config.js

@@ -30,6 +30,13 @@ if (!ACCESS_KEY || !/^\d{6}$/.test(ACCESS_KEY)) {
   process.exit(1);
 }
 
+// 管理者キー（任意）。入力すると管理者として入室でき、ハンドレンジ学習ページが見える
+const ADMIN_KEY = process.env.ADMIN_KEY || null;
+if (ADMIN_KEY && (!/^\d{6}$/.test(ADMIN_KEY) || ADMIN_KEY === ACCESS_KEY)) {
+  console.error('[config] ADMIN_KEY は ACCESS_KEY と異なる6桁の数字にしてください。');
+  process.exit(1);
+}
+
 module.exports = {
   PORT: Number(process.env.PORT) || 4671,
 
@@ -43,6 +50,7 @@ module.exports = {
 
   // 参加キー（6桁の数字）。.env の ACCESS_KEY
   ACCESS_KEY,
+  ADMIN_KEY,
 
   // 1テーブルの最大着席数
   MAX_SEATS: 9,