✨ add New Window button to playground

[guilhermevrs]

Motivation

Enable testing multi-window scenarios in the playground app.

Changes

• Added a "New Window" button to the playground UI.
• Wired up open-new-window IPC channel: renderer -> preload -> main.
• Main process spawns a new BrowserWindow loading the same index.html (same UI for now).

Test instructions

• cd playground && yarn dev
• Click "New Window" in the SDK Actions section; a second window should open loading the same playground UI.

Checklist

• Tested locally (playground)
• Added unit tests for this change.
• Added e2e/integration tests for this change.
• Updated related documentation.
• Agentic code review findings addressed or explicitly dismissed.