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>

public/table.html

@@ -100,8 +100,8 @@
   </div>
 
   <div class="center-message" id="not-found" hidden>
-    <h2>テーブルが見つかりません</h2>
-    <p>終了したか、URLが間違っている可能性があります。</p>
+    <h2 id="not-found-title">テーブルが見つかりません</h2>
+    <p id="not-found-text">終了したか、URLが間違っている可能性があります。</p>
     <a class="btn btn-blue" href="/">ロビーへ戻る</a>
   </div>