Improve connection-state reporting in list_browsers

[forier]

Problem

list_browsers only reports connected: true/false based on Puppeteer's browser.connected property. This doesn't catch all disconnection scenarios (e.g. Chrome crashed but Puppeteer hasn't detected it yet).

Expected behavior

• Actively verify the connection by pinging the browser before reporting status
• Report additional info: browser URL/port, number of open pages, Chrome version
• Detect and mark stale connections

Suggested output format

```

Connected browsers

• default: connected (port 9222, 5 pages, Chrome 146.0) [selected]
• test-user: connected (port 9223, 2 pages, Chrome 146.0)
• staging: disconnected (port 9224, last seen 5m ago)
  ```

Files to modify

• build/src/browser.js — listBrowsers() function
• build/src/tools/multibrowser.js — list_browsers handler