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.
Four components duplicated the watch -> nextTick -> listenToClick -> runTikz
shape. Collapse to useMarkdownPostRender, which also accepts optional flags
for Mermaid, Shikiji and image hydration so StackedNote no longer needs its
own inline conditional chain.
