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>

.claude/launch.json

@@ -6,6 +6,13 @@
       "runtimeExecutable": "node",
       "runtimeArgs": ["server/index.js"],
       "port": 4671
+    },
+    {
+      "name": "poker-test",
+      "runtimeExecutable": "node",
+      "runtimeArgs": ["server/index.js"],
+      "env": { "ACCESS_KEY": "000000", "ADMIN_KEY": "000001", "PORT": "4672" },
+      "port": 4672
     }
   ]
 }