gpui: Fix SVG renderer not rendering text when system fonts are unavailable

[Dnreikronos]

Closes #51466

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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.

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[Dnreikronos]

The problem

Mermaid diagrams in Zed's markdown preview showed up as empty boxes. No text at all. The logs were full of this:

No match for '"Inter", "ui-sans-serif", "system-ui", sans-serif' font-family.

Two things going wrong here. On some Linux setups, fontconfig doesn't resolve fonts properly, leaving the database empty or stuck with defaults that aren't actually installed. On top of that, fontdb doesn't understand CSS keywords like system-ui or ui-sans-serif, which is exactly what Mermaid puts in its SVG output. So even when the right fonts are on the machine, they weren't being found.

What changed (crates/gpui/src/svg_renderer.rs)

First, we load IBM Plex Sans and Lilex (both already ship with Zed) into the SVG font database via load_bundled_fonts(). This way there's always at least one sans-serif and one monospace font available.

Second, we remap generic font families. fontdb points sans-serif at Arial, monospace at Courier New, and so on. Those don't exist on most Linux systems. fix_generic_font_families() redirects these to the bundled fonts when the defaults fail to resolve.

Third, we added a last-resort fallback in the font resolver. If nothing matches (e.g. system-ui), we pick the first available font instead of returning nothing.

Impact

Mermaid text renders correctly now, even on systems with broken font resolution. System fonts are still loaded and preferred; the bundled fonts only step in as fallbacks. The font database is built once (LazyLock) and cloned, so there's no extra I/O cost. No new dependencies since both fonts already ship with Zed.

[zed-industries-bot]

	Warnings
⚠️	This PR is missing release notes. Please add a "Release Notes" section that describes the change: Release Notes: - Added/Fixed/Improved ... If your change is not user-facing, you can use "N/A" for the entry: Release Notes: - N/A

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[smitbarmase]

Thanks!

[qnfm]

Thanks, finally landed in upstream. I don't need to maintain a cherry-picked branch.