Optional Extra wide Web page layouts with a configurable breakpoint

Add an optional Extra wide layout to individual WebPages, with a configurable browser-width threshold. This lets developers design a dashboard with two charts per row on a laptop and three per row on a wider browser window, while retaining separate layouts for smaller screens.

Keep Compact, Medium and Wide as the default. Extra wide uses the same 12-column grid as Wide and initially reuses the Wide arrangement, so developers only maintain another layout when they choose to customise it. Offer a clear way to return to using Wide.

The proposed default threshold is 1440 CSS pixels. Developers can choose a different supported threshold for each page. Changing the layout settings is saved with the project and undoable. Preview width and zoom remain viewing controls.

This is separate from Full width pages: Full width controls how much browser space the page uses; Extra wide controls how its controls are arranged.