Closes #51466

 Before you mark this PR as ready for review, make sure that you have:

- [x] Added a solid test coverage and/or screenshots from doing manual
testing

- [x] Done a self-review taking into account security and performance
aspects

- [ ] Aligned any UI changes with the [UI
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)

 Release Notes:

- Fixed mermaid diagrams not showing text in markdown preview by
bundling fallback fonts and fixing generic font family resolution in the
SVG renderer.