Implement Texas Hold'em play: dealing, betting, side pots, showdown

- Hand engine (server/hand.js): crypto-shuffled deck kept server-side,
  blinds (heads-up button = SB), min-raise rules incl. incomplete all-in,
  uncalled bet refunds, side pots, split pots, all-in runout
- Hand evaluator (server/cards.js) and node:test suite (npm test)
- Table flow: hands auto-continue after host START, 30s action clock
  (timeout = check/fold + sit out), sit out / sit in, rebuy, mid-hand leave
  folds and keeps the seat reserved until the hand ends, pause freezes all clocks
- Only each player's own hole cards are sent until a showdown/runout reveal;
  all actions validated server-side
- Play UI: board, hole cards, bets, action tags, turn timer, raise panel with
  presets, showdown results with winning cards highlighted, compact 7+ layout
- Remove the host crown and the temporary "next phase" control

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

server/cards.js

@@ -0,0 +1,96 @@
+'use strict';
+
+const crypto = require('crypto');
+
+// カードは "As"（スペードのA）, "Td"（ダイヤの10）のような2文字
+const RANKS = '23456789TJQKA';
+const SUITS = 'shdc';
+
+// 暗号論的乱数で Fisher-Yates シャッフル（山札はサーバの外に出さない）
+function newDeck() {
+  const deck = [];
+  for (const r of RANKS) for (const s of SUITS) deck.push(r + s);
+  for (let i = deck.length - 1; i > 0; i--) {
+    const j = crypto.randomInt(i + 1);
+    [deck[i], deck[j]] = [deck[j], deck[i]];
+  }
+  return deck;
+}
+
+const rankValue = (card) => RANKS.indexOf(card[0]) + 2;
+
+// 5枚の役の強さ。配列を先頭から比較して大きいほうが強い
+// [役の種類(0〜8), 比較用の数字...]
+function score5(cards) {
+  const vals = cards.map(rankValue).sort((a, b) => b - a);
+  const flush = cards.every((c) => c[1] === cards[0][1]);
+
+  let straightHigh = 0;
+  if (new Set(vals).size === 5) {
+    if (vals[0] - vals[4] === 4) straightHigh = vals[0];
+    else if (vals[0] === 14 && vals[1] === 5) straightHigh = 5; // A-2-3-4-5
+  }
+  if (straightHigh && flush) return [8, straightHigh];
+
+  // 枚数の多い順、同じ枚数なら数字の大きい順
+  const counts = new Map();
+  for (const v of vals) counts.set(v, (counts.get(v) || 0) + 1);
+  const groups = [...counts].sort((a, b) => b[1] - a[1] || b[0] - a[0]);
+  const byGroup = groups.map((g) => g[0]);
+
+  if (groups[0][1] === 4) return [7, ...byGroup];
+  if (groups[0][1] === 3 && groups[1][1] === 2) return [6, ...byGroup];
+  if (flush) return [5, ...vals];
+  if (straightHigh) return [4, straightHigh];
+  if (groups[0][1] === 3) return [3, ...byGroup];
+  if (groups[0][1] === 2 && groups[1][1] === 2) return [2, ...byGroup];
+  if (groups[0][1] === 2) return [1, ...byGroup];
+  return [0, ...vals];
+}
+
+function compareScore(a, b) {
+  for (let i = 0; i < Math.max(a.length, b.length); i++) {
+    const d = (a[i] || 0) - (b[i] || 0);
+    if (d) return d;
+  }
+  return 0;
+}
+
+const HAND_NAMES = [
+  'ハイカード',
+  'ワンペア',
+  'ツーペア',
+  'スリーカード',
+  'ストレート',
+  'フラッシュ',
+  'フルハウス',
+  'フォーカード',
+  'ストレートフラッシュ',
+];
+
+function handName(score) {
+  if (score[0] === 8 && score[1] === 14) return 'ロイヤルフラッシュ';
+  return HAND_NAMES[score[0]];
+}
+
+// 5〜7枚から最も強い5枚を選ぶ
+function bestHand(cards) {
+  let best = null;
+  const n = cards.length;
+  const pick = (start, chosen) => {
+    if (chosen.length === 5) {
+      const score = score5(chosen);
+      if (!best || compareScore(score, best.score) > 0) best = { score, cards: chosen.slice() };
+      return;
+    }
+    for (let i = start; i <= n - (5 - chosen.length); i++) {
+      chosen.push(cards[i]);
+      pick(i + 1, chosen);
+      chosen.pop();
+    }
+  };
+  pick(0, []);
+  return best && { score: best.score, cards: best.cards, name: handName(best.score) };
+}
+
+module.exports = { newDeck, score5, compareScore, bestHand, handName };