fix: use monospace font in Markdown previews

[Radiummm]

Summary

• use the existing monospace font stack in rendered Markdown file previews
• keep the change scoped to the file viewer so chat typography is unchanged
• add regression coverage for the preview style

Tests

• npm test (845 passed)
• node_modules/.bin/tsc --noEmit
• npm run lint

Closes #638

[agegr]

Plain-text code blocks in Markdown file previews already use the existing --font-mono stack on main, including fences without a language and those marked text or plaintext (CodeBlock). ASCII diagrams should be placed in code blocks so their spacing and line breaks are preserved.

This PR changes the font of the entire preview, including ordinary headings and prose. We prefer to keep proportional fonts for that content, so we are closing this PR. Changing the body font also does not address whitespace collapsing outside code blocks. A minimal Markdown example reproducing #638 inside a code block would help us identify and fix the underlying rendering issue.