feat(markdown): add hover download buttons on rendered SVGs

Attach SVG and PNG (×2) download buttons to every rendered SVG (TikZ,
Mermaid, …) once post-render finishes. Exports always use the original
colors with a white background baked in, regardless of theme.

src/hooks/useMarkdown.hook.ts

@@ -174,7 +174,7 @@ export const useShikiji = (): Promise<void> => {
 
 let mermaidInitialized = false
 
-export const runMermaid = (querySelector: string) => {
+export const runMermaid = (querySelector: string): Promise<void> => {
   if (!mermaidInitialized) {
     mermaidInitialized = true
     mermaid.initialize({
@@ -184,7 +184,7 @@ export const runMermaid = (querySelector: string) => {
     })
   }
 
-  mermaid.run({
+  return mermaid.run({
     querySelector
   })
 }