fix(viewer): add spacing between callout and columns blocks

[stan373]

Description

In the CLI Viewer, a callout block placed immediately above a columns block has no visible vertical spacing. The callout's bottom border touches the top edge of the columns container, so the two blocks look visually stuck together.

Expected behavior

Add a consistent vertical margin or block gap between adjacent callout and columns blocks, matching the spacing used between other document blocks.

Reproduction

1. Open a document in the CLI Viewer.
2. Place a callout block immediately before a two-column block.
3. Observe the boundary between the callout and the columns container.

Environment

• Univer CLI: 0.5.0-insider.20260826-4d4b6be
• Surface: local CLI Viewer