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/hooks/useImageLightbox.hook.ts

@@ -0,0 +1,19 @@
+import { ref } from "vue"
+
+const isOpen = ref(false)
+const src = ref("")
+const alt = ref("")
+
+export const useImageLightbox = () => {
+  const open = (imageSrc: string, imageAlt = "") => {
+    src.value = imageSrc
+    alt.value = imageAlt
+    isOpen.value = true
+  }
+
+  const close = () => {
+    isOpen.value = false
+  }
+
+  return { isOpen, src, alt, open, close }
+}