feat: fullscreen image lightbox on image/diagram click

Click a note image or a mermaid/tikz diagram to view it fullscreen over
a blurred dim backdrop, with a fade/scale animation and rounded white
frame. Works on desktop and mobile; diagram SVGs are serialized to a
data URL via svgToDataUrl. Closes on click or Escape.

src/utils/svgDownload.ts

@@ -101,6 +101,9 @@ const buildExportableSvgString = (svg: SVGSVGElement): string => {
   return new XMLSerializer().serializeToString(clone)
 }
 
+export const svgToDataUrl = (svg: SVGSVGElement): string =>
+  `data:image/svg+xml;charset=utf-8,${encodeURIComponent(buildExportableSvgString(svg))}`
+
 const triggerBlobDownload = (blob: Blob, filename: string): void => {
   const url = URL.createObjectURL(blob)
   const a = document.createElement("a")