Mermaid SVG font family is not configurable, causing unfixable CJK font fallback

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As shown above, Zed hard-codes Mermaid's font family to ui_font_family. As a result, users who rely on ui_font_fallback for correct CJK rendering cannot fix Mermaid's font rendering through settings.json alone. Thus the default font fallback behavior in most operating systems will prioritize Japanese glyphs for CJK text unless the fallback fonts are configured correctly—a common CJK font fallback issue, especially affects systems set to Singapore timezone.

At its core, this is a limitation in Zed's current font fallback implementation that prevents users from configuring the fallback behavior. For CJK users, this is not merely a matter of being unable to achieve a preferred customization; it means that incorrect font fallback cannot be corrected at all.

I've written a PR for this.

(Hope my Chinglish doesn't sound too GPTish. Trust me bro it's human-generated.)