refactor(markdown): extract post-render effects into composable

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.

src/hooks/useMarkdownPostRender.hook.ts

@@ -0,0 +1,54 @@
+import { nextTick, type Ref, toValue, watch } from "vue"
+
+import { useImages } from "@/hooks/useImages.hook"
+import {
+  runMermaid,
+  runTikz,
+  useShikiji
+} from "@/hooks/useMarkdown.hook"
+
+interface MarkdownPostRenderOptions {
+  onReady?: () => void
+  tikz?: boolean
+  mermaid?: () => boolean
+  shikiji?: () => boolean
+  images?: () => string | null | undefined
+  triggers?: Ref<unknown>[]
+}
+
+export const useMarkdownPostRender = (
+  contentRef: Ref<unknown>,
+  scopeSelector: () => string,
+  options: MarkdownPostRenderOptions = {}
+) => {
+  const sources = [contentRef, ...(options.triggers ?? [])]
+
+  watch(
+    sources,
+    async () => {
+      if (!toValue(contentRef)) return
+      await nextTick()
+      options.onReady?.()
+
+      const scope = scopeSelector()
+
+      if (options.tikz) {
+        void runTikz(`${scope} .tikz`)
+      }
+
+      if (options.mermaid?.()) {
+        runMermaid(`${scope} .mermaid`)
+      }
+
+      if (options.shikiji?.()) {
+        void useShikiji()
+      }
+
+      const imagesSha = options.images?.()
+      if (imagesSha) {
+        useImages(imagesSha)
+      }
+    },
+    { immediate: true }
+  )
+}