Add CPU opponents and a table type selector (cash only / friends / CPU)

- Create screen: choose キャッシュオンリー (real cards, chips only), 友人戦
  (server deals and settles) or CPU戦 (友人戦 plus 1–8 CPU players); replaces
  the card toggle. The lobby shows each table's type
- Poker AI written from scratch (server/ai.js, no libraries): fast 7-card
  evaluator (checked against bestHand), Monte Carlo equity, and decisions from
  hand rank, pot odds, raise count, opponents and a per-CPU style (aggression,
  bluff rate); continuation bets via the last aggressor. It never sees other
  players' hole cards
- CPU seats: think time of 0.7–1.8s (faster when no human is left in the hand),
  automatic rebuy, never the host, no new hand unless a human is playing,
  re-think after pause
- Play UI: robot avatars and a "考えています…" status for CPU turns
- Tests for the evaluator, equity, legality of AI actions and CPU tables

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

server/config.js

@@ -67,6 +67,12 @@ module.exports = {
   SHOWDOWN_MS: 4000,
   CARD_SHOWDOWN_MS: 6000,
   FOLD_WIN_MS: 2500,
+  // CPU の考える時間（ランダム）。人間がハンドに残っていないときは BOT_FAST_MS
+  BOT_THINK_MIN_MS: 700,
+  BOT_THINK_MAX_MS: 1800,
+  BOT_FAST_MS: 400,
+  // CPU が勝率を見積もるときのシミュレーション回数
+  BOT_ITERATIONS: 800,
   // 参加者が2人揃ってから次のハンドを始めるまで
   NEXT_HAND_DELAY_MS: 2000,