## Summary

Wide Mermaid diagrams in Markdown Preview were forced through
`max_w_full()`, so both width and height shrank to the pane and text
became unreadable/blurry. There was no horizontal pan.

Markdown Preview already sets `code_block_overflow_x_scroll: true` for
fenced code blocks. Mermaid ignored that flag.

This change gates layout on that style flag:

- **Preview** (`code_block_overflow_x_scroll`): keep the raster at
intrinsic size and enable `overflow_x_scroll` (with
`restrict_scroll_to_axis`) so wide diagrams stay readable and can be
panned horizontally.
- **Compact UIs** (e.g. agent): keep previous `max_w_full()` fit-to-pane
behavior.

Fixes #61051

## Test plan

- [ ] Open a markdown file with a wide ` ```mermaid ` flowchart (e.g.
multi-column TB graph) in Markdown Preview
- [ ] Confirm diagram is not crushed to pane width; text remains legible
at default scale
- [ ] Confirm horizontal scroll/pan works on the diagram without
stealing vertical page scroll
- [ ] Confirm compact surfaces that leave `code_block_overflow_x_scroll`
off still fit diagrams to width
- [ ] Confirm mermaid scale fence info still works (` ```mermaid 150 `)
- [ ] Confirm Preview/Code tabs and copy button still work

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Release Notes:

- Fixed wide Mermaid diagrams in Markdown Preview being squashed and
illegible