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>

admin/range.html

@@ -0,0 +1,75 @@
+<!doctype html>
+<html lang="ja">
+<head>
+  <meta charset="utf-8">
+  <meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover">
+  <meta name="theme-color" content="#24493b">
+  <meta name="robots" content="noindex, nofollow">
+  <title>ハンドレンジ - Hold'em Night</title>
+  <link rel="icon" href="/assets/favicon.ico">
+  <link rel="stylesheet" href="/assets/style.css">
+  <link rel="stylesheet" href="/range/files/range.css">
+</head>
+<body>
+  <main class="wrap range-wrap">
+    <div class="topbar">
+      <a class="icon-btn" href="/" aria-label="ロビーへ戻る">←</a>
+      <h1>ハンドレンジ</h1>
+    </div>
+
+    <div class="seg" id="tabs">
+      <button type="button" class="seg-btn active" data-view="range">レンジ表</button>
+      <button type="button" class="seg-btn" data-view="quiz">クイズ</button>
+    </div>
+
+    <!-- レンジ表 -->
+    <section id="view-range">
+      <div class="range-matrix" id="matrix" aria-label="13×13 ハンドレンジ表"></div>
+      <div class="range-legend" id="legend"></div>
+
+      <div class="card mt-12" id="selected">
+        <div class="sel-hand">—</div>
+        <div class="hint">セルをタップすると分類を確認できます</div>
+      </div>
+
+      <button type="button" class="btn btn-orange btn-block mt-12" id="start-from-range">クイズを始める</button>
+    </section>
+
+    <!-- クイズ -->
+    <section id="view-quiz" hidden>
+      <div class="label">問題数（全169ハンドからランダム）</div>
+      <div class="seg" id="count">
+        <button type="button" class="seg-btn active" data-count="10">10問</button>
+        <button type="button" class="seg-btn" data-count="20">20問</button>
+        <button type="button" class="seg-btn" data-count="0">全問</button>
+      </div>
+
+      <div class="quiz-progress"><i id="progress-bar"></i></div>
+      <div class="quiz-stats">
+        <span id="q-count">開始前</span>
+        <span id="q-score">正解 0 / 0</span>
+        <span id="q-accuracy">正答率 --</span>
+      </div>
+
+      <div class="card quiz-hand">
+        <div class="quiz-cards">
+          <img id="card-1" alt="" hidden>
+          <img id="card-2" alt="" hidden>
+        </div>
+        <div class="quiz-hand-name" id="q-hand">?</div>
+        <div class="hint" id="q-prompt">「開始」を押してください</div>
+      </div>
+
+      <div class="quiz-answers" id="answers"></div>
+      <div class="quiz-feedback" id="feedback" aria-live="polite" hidden></div>
+
+      <div class="row mt-12">
+        <button type="button" class="btn btn-dark" id="start-quiz">開始</button>
+        <button type="button" class="btn btn-blue" id="next-question" disabled>次の問題 →</button>
+      </div>
+    </section>
+  </main>
+
+  <script src="/range/files/range.js"></script>
+</body>
+</html>