Is your feature request related to a problem? Please describe.

Allow users of the Tiled Gallery to change the block's layout to Mosaic. The criteria of done here is that the block's output (as seen when previewing a post in the main app) should match the web. The output should be equal when comparing the HTML generated on mobile and web, and should also be visually the same (i.e. the styles should match).

Describe the solution you'd like
Unlike the block's other layout, no className is applied to the block's HTML when the Mosaic layout is chosen. Presumably this is because Mosaic is the default layout.

It's worth noting that unlike the Square layout, the Mosaic layout has no Columns setting in its settings bottom sheet nor columns attribute in its HTML.

Additional context
This issue captures the Circle layout's basic implementation and explicitly excludes the in-editor preview of the Circle layout, which will be tracked in another issue.