fix(markdown): embed used @font-face rules in exported SVGs

The exported SVG carries font-family="cmr7"/"cmsy7"/… attributes but
no @font-face, so standalone viewers fall back to system fonts. That
scatters TikZ glyphs (precomputed x positions assume Computer Modern
metrics) and turns cmsy7 minus signs into ¡ (custom symbol encoding).
Inline only the rules for font-families the SVG actually uses, and
await document.fonts.ready before rasterizing to PNG.

src/utils/svgDownload.ts

@@ -3,6 +3,55 @@ const SVG_NS = "http://www.w3.org/2000/svg"
 
 const TABLER_DOWNLOAD_ICON = `<svg xmlns="${SVG_NS}" class="icon icon-tabler icon-tabler-download" width="14" height="14" viewBox="0 0 24 24" stroke-width="2" stroke="currentColor" fill="none" stroke-linecap="round" stroke-linejoin="round"><path stroke="none" d="M0 0h24v24H0z" fill="none"/><path d="M4 17v2a2 2 0 0 0 2 2h12a2 2 0 0 0 2 -2v-2"/><path d="M7 11l5 5l5 -5"/><path d="M12 4l0 12"/></svg>`
 
+const stripQuotes = (s: string): string => s.trim().replace(/^["']|["']$/g, "")
+
+const collectFontFamilies = (svg: SVGSVGElement): Set<string> => {
+  const families = new Set<string>()
+  const collect = (raw: string | null) => {
+    if (!raw) return
+    raw.split(",").forEach((part) => {
+      const cleaned = stripQuotes(part)
+      if (cleaned) families.add(cleaned)
+    })
+  }
+  collect(svg.getAttribute("font-family"))
+  svg.querySelectorAll<Element>("[font-family]").forEach((el) =>
+    collect(el.getAttribute("font-family"))
+  )
+  return families
+}
+
+const isFontFaceRule = (rule: CSSRule): boolean => {
+  if (typeof CSSFontFaceRule !== "undefined" && rule instanceof CSSFontFaceRule)
+    return true
+  return rule.cssText.trimStart().startsWith("@font-face")
+}
+
+const collectFontFaceCss = (families: Set<string>): string => {
+  if (families.size === 0) return ""
+  const parts: string[] = []
+  for (const sheet of Array.from(document.styleSheets)) {
+    let rules: CSSRuleList | null = null
+    try {
+      rules = sheet.cssRules
+    } catch {
+      continue
+    }
+    if (!rules) continue
+    for (const rule of Array.from(rules)) {
+      if (!isFontFaceRule(rule)) continue
+      const styleRule = rule as CSSFontFaceRule
+      const family = stripQuotes(
+        styleRule.style?.getPropertyValue("font-family") ?? ""
+      )
+      if (family && families.has(family)) {
+        parts.push(rule.cssText)
+      }
+    }
+  }
+  return parts.join("\n")
+}
+
 const getSvgPixelSize = (svg: SVGSVGElement): { width: number; height: number } => {
   const viewBox = svg.viewBox?.baseVal
   if (viewBox && viewBox.width > 0 && viewBox.height > 0) {
@@ -36,6 +85,14 @@ const buildExportableSvgString = (svg: SVGSVGElement): string => {
   bg.setAttribute("fill", "#ffffff")
   clone.insertBefore(bg, clone.firstChild)
 
+  const fontFaceCss = collectFontFaceCss(collectFontFamilies(svg))
+  if (fontFaceCss) {
+    const style = document.createElementNS(SVG_NS, "style")
+    style.setAttribute("type", "text/css")
+    style.textContent = fontFaceCss
+    clone.insertBefore(style, clone.firstChild)
+  }
+
   return new XMLSerializer().serializeToString(clone)
 }
 
@@ -67,6 +124,9 @@ const downloadAsPng = async (
   const svgUrl = URL.createObjectURL(svgBlob)
 
   try {
+    if (document.fonts?.ready) {
+      await document.fonts.ready
+    }
     const img = new Image()
     img.crossOrigin = "anonymous"
     await new Promise<void>((resolve, reject) => {