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>

server/tables.js

@@ -152,6 +152,11 @@ class Table {
     return true;
   }
 
+  // CPU戦で人間が全員退席したら、テーブルを残しておく意味がない
+  shouldClose() {
+    return this.mode === 'cpu' && !this.seats.some((s) => s && !s.left && !s.bot);
+  }
+
   removeLeftSeats() {
     this.seats = this.seats.map((s) => (s && s.left ? null : s));
   }