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/assets/table.js

@@ -52,6 +52,7 @@
     });
     socket.on('disconnect', () => { $('conn-lost').hidden = false; });
     socket.on('connect_error', () => { $('conn-lost').hidden = false; });
+    socket.on('table:closed', () => showNotFound(true));
     socket.on('table:state', (s) => {
       state = s;
       pending = false;
@@ -90,11 +91,22 @@
     render();
   }
 
-  function showNotFound() {
+  // closed: プレイヤーが全員退席してテーブルが閉じられた
+  function showNotFound(closed) {
+    if (closed) {
+      $('not-found-title').textContent = 'テーブルが終了しました';
+      $('not-found-text').textContent = 'プレイヤーが全員退席したため、テーブルを閉じました。';
+    }
     $('main').hidden = true;
     $('action-bar').hidden = true;
     $('not-found').hidden = false;
-    if (socket) socket.disconnect();
+    // 自分から切断するので「接続が切れました」は出さない
+    if (socket) {
+      socket.off('disconnect');
+      socket.off('connect_error');
+      socket.disconnect();
+    }
+    $('conn-lost').hidden = true;
   }
 
   const nameOf = (seat) => (state.seats[seat] ? state.seats[seat].name : '退席者');