Add browser smoke check for dashboard layout

[Martinnn674]

The dashboard now has a local-access panel, but we only assert the markup and run a Playwright measurement manually.

Add a small automated browser smoke check that:

• starts the app in mock mode
• opens the dashboard at desktop and mobile widths
• verifies the Local Access panel is visible
• fails on horizontal overflow in the dashboard surface

This should stay lightweight and not require a real OpenClaw install.

[Martinnn674]

Implemented by #6 with a Playwright dashboard smoke check covering mock-mode desktop/mobile layout and horizontal overflow.