Contained and full width Web page layouts with adjustable designer preview width

Add a Page width setting to WebPages with Contained and Full width options. Contained preserves the current centred page frame with its 1200px maximum width. Full width uses the available browser width while retaining the current page padding.

This lets developers use more horizontal space for tables, charts and dashboards while keeping forms and other pages contained where appropriate. The setting is independent for each page, saved with the project and undoable. Contained remains the default for both new pages and existing projects.

Keep the existing Compact, Medium and Wide layouts, breakpoint thresholds and 4/8/12-column grids. Full width gives the existing columns more room without adding another breakpoint or requiring another authored layout.

Include adjustable designer preview width so developers can inspect the selected layout at different browser widths, including wider Wide previews such as 1440px and 1920px. Keep the existing breakpoint buttons and provide a fit-to-view option for previews wider than the designer workspace. Preview width and zoom are local viewing state and do not change the saved page layout or add document undo entries.