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>

public/assets/lobby.js

@@ -36,6 +36,8 @@
     if (btn && requireName()) location.href = '/table/' + encodeURIComponent(btn.dataset.join);
   });
 
+  const MODE_LABEL = { chips: '💰 キャッシュオンリー', friends: '🃏 友人戦', cpu: '🤖 CPU戦' };
+
   function phaseBadge(t) {
     if (t.paused) return '<span class="badge paused">PAUSED</span>';
     if (t.phase === 'WAITING') return '<span class="badge">WAITING</span>';
@@ -61,7 +63,7 @@
                 <span>開始 <b>${chips(t.startingStack)}</b></span>
                 <span class="gold">Lv.${t.level} <b class="gold">${chips(t.smallBlind)}/${chips(t.bigBlind)}</b></span>
                 <span>${timer}</span>
-                ${t.cardsEnabled ? '<span>🃏 カードあり</span>' : ''}
+                <span>${MODE_LABEL[t.mode] || ''}</span>
               </div>
             </div>
             <button class="btn btn-blue btn-sm" data-join="${esc(t.id)}" ${full ? 'title="満席（観戦のみ）"' : ''}>