Replace native confirm() with an in-site confirmation dialog

Add App.confirm() in common.js: a promise-based dialog styled like the
site's modals, with a title, multi-line message and custom button labels.
Destructive actions use a red OK button and focus Cancel by default;
Escape and backdrop clicks cancel, and Tab stays inside the dialog.

Use it for folding when a check is possible, leaving the table,
confirming showdown winners and deleting tables from the lobby. After the
dialog closes, re-check that the action is still valid (turn not passed,
pot not already awarded, table still listed) before sending it.

Also add a "poker-test" launch config that runs on port 4672 with the
example keys from .env.example, for local browser testing.

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

public/assets/lobby.js

@@ -47,12 +47,18 @@
     if (btn && requireName()) location.href = '/table/' + encodeURIComponent(btn.dataset.join);
   });
 
-  function deleteTable(id, btn) {
+  async function deleteTable(id, btn) {
     const t = tables && tables.find((x) => x.id === id);
     if (!t) return;
     const players = t.playerCount ? `
 着席中の ${t.playerCount} 人はテーブルから退出させられます。` : '';
-    if (!confirm(`「${t.name}」を削除しますか？${players}`)) return;
+    const ok = await App.confirm({
+      title: 'テーブルを削除',
+      message: `「${t.name}」を削除しますか？${players}`,
+      ok: '削除する',
+      danger: true,
+    });
+    if (!ok || !document.contains(btn)) return;
     btn.disabled = true;
     App.api('DELETE', '/api/tables/' + encodeURIComponent(id))
       .then(() => App.toast('テーブルを削除しました'))