Close CPU tables as soon as every human player leaves

- When the last human presses leave on a CPU table (even mid-hand), the table
  is deleted right away instead of lingering with CPUs only; friends / cash-only
  tables keep the existing 30-minute idle cleanup
- Spectators still watching get a "テーブルが終了しました" notice, and the lobby
  updates immediately
- Don't show the "接続が切れました" banner when the page disconnects on purpose
  (closed or missing table)
- Test for the close condition; README notes the behavior

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

test/engine.test.js

@@ -698,3 +698,30 @@ test('CPU戦: CPU が席に着き、ホストは人間のまま、CPU は自動
   t.join(T(5), 'friend');
   assert.equal(t.isHost(T(5)), true);
 });
+
+test('CPU戦: 人間が全員退席したらテーブルを閉じる（ハンド中でも）', () => {
+  const opts = {
+    id: 'c',
+    name: 'cpu',
+    startingStack: 1000,
+    smallBlind: 10,
+    bigBlind: 20,
+    blindTimerEnabled: false,
+    blindIntervalMin: 10,
+    hostToken: T(0),
+  };
+  const t = new Table({ ...opts, mode: 'cpu', cpuCount: 2 });
+  t.join(T(0), 'me');
+  t.join(T(5), 'friend');
+  t.start(T(0), 0);
+  t.leave(T(0));
+  assert.equal(t.shouldClose(), false); // まだ友達がいる
+  t.leave(T(5));
+  assert.equal(t.shouldClose(), true);
+
+  // CPU戦以外は閉じない（これまでどおり、誰も接続していない状態が続いたら削除）
+  const f = new Table({ ...opts, mode: 'friends' });
+  f.join(T(0), 'me');
+  f.leave(T(0));
+  assert.equal(f.shouldClose(), false);
+});