refactor(desktop): route preview-pane mermaid fences through shared embeds registry

Drop the duplicate mermaid-block.tsx (own mermaid.initialize + render path,
theme frozen at first load) and wire preview-file.tsx's MarkdownCode through
the existing RichCodeBlock registry from #52935 instead. One mermaid init
path, theme-flip re-init, Zoomable + copy-as-PNG, RichBoundary error
fallback — and the preview pane gets svg fences for free. Shiki block stays
as the fallback for all other languages.

apps/desktop/src/app/chat/right-rail/preview-file.tsx

@@ -6,7 +6,7 @@ import type {
   MouseEvent as ReactMouseEvent,
   ReactNode
 } from 'react'
-import { Fragment, lazy, Suspense, useCallback, useEffect, useMemo, useRef, useState } from 'react'
+import { Fragment, useCallback, useEffect, useMemo, useRef, useState } from 'react'
 import ShikiHighlighter from 'react-shiki'
 import { Streamdown } from 'streamdown'
 
@@ -14,6 +14,7 @@ import { requestComposerFocus, requestComposerInsertRefs } from '@/app/chat/comp
 import { droppedFileInlineRef } from '@/app/chat/composer/inline-refs'
 import { HERMES_PATHS_MIME } from '@/app/chat/hooks/use-composer-actions'
 import { isAddSelectionShortcut } from '@/app/right-sidebar/terminal/selection'
+import { RichCodeBlock } from '@/components/assistant-ui/embeds'
 import { CodeEditor } from '@/components/chat/code-editor'
 import { FileDiffPanel } from '@/components/chat/diff-lines'
 import { chunkTextLines, useFixedRowWindow } from '@/components/chat/fixed-row-window'
@@ -34,10 +35,6 @@ import { setPreviewDirty } from '@/store/preview-edit'
 import { $currentCwd } from '@/store/session'
 import { notifyWorkspaceChanged } from '@/store/workspace-events'
 
-const MermaidDiagram = lazy(() =>
-  import('@/components/chat/mermaid-block').then(m => ({ default: m.MermaidBlock }))
-)
-
 const SHIKI_THEME = { dark: 'github-dark-default', light: 'github-light-default' } as const
 const TEXT_PREVIEW_MAX_BYTES = 512 * 1024
 const SOURCE_CHUNK_LINES = 200
@@ -295,17 +292,9 @@ function MarkdownCode({ className, children, ...props }: ComponentProps<'code'>)
     )
   }
 
-  if (language === 'mermaid') {
-    return (
-      <Suspense
-        fallback={<div className="my-3 text-xs text-muted-foreground">Loading diagram…</div>}
-      >
-        <MermaidDiagram chart={String(children).replace(/\n$/, '')} />
-      </Suspense>
-    )
-  }
+  const code = String(children).replace(/\n$/, '')
 
-  return (
+  const highlighted = (
     <ShikiHighlighter
       addDefaultStyles={false}
       as="div"
@@ -315,9 +304,13 @@ function MarkdownCode({ className, children, ...props }: ComponentProps<'code'>)
       showLanguage={false}
       theme={SHIKI_THEME}
     >
-      {String(children).replace(/\n$/, '')}
+      {code}
     </ShikiHighlighter>
   )
+
+  // ```mermaid / ```svg fences route to the shared lazy renderers (same
+  // registry the chat transcript uses); everything else stays on Shiki.
+  return <RichCodeBlock code={code} fallback={highlighted} language={language} />
 }
 
 const MARKDOWN_COMPONENTS = {

apps/desktop/src/components/chat/mermaid-block.tsx

@@ -1,84 +0,0 @@
-import { useEffect, useRef, useState } from 'react'
-
-let mermaidPromise: Promise<typeof import('mermaid')> | null = null
-
-function loadMermaid() {
-  if (!mermaidPromise) {
-    mermaidPromise = import('mermaid').then(async mod => {
-      const m = mod.default
-
-      m.initialize({
-        securityLevel: 'strict',
-        startOnLoad: false,
-        theme: document.documentElement.classList.contains('dark') ? 'dark' : 'default'
-      })
-
-      return mod
-    })
-  }
-
-  return mermaidPromise
-}
-
-interface MermaidBlockProps {
-  chart: string
-}
-
-export function MermaidBlock({ chart }: MermaidBlockProps) {
-  const containerRef = useRef<HTMLDivElement>(null)
-  const [error, setError] = useState<string | null>(null)
-  const [svg, setSvg] = useState<string | null>(null)
-
-  useEffect(() => {
-    let cancelled = false
-
-    async function render() {
-      try {
-        const { default: mermaid } = await loadMermaid()
-        const id = `mermaid-${Math.random().toString(36).slice(2, 10)}`
-        const { svg: rendered } = await mermaid.render(id, chart)
-
-        if (!cancelled) {
-          setSvg(rendered)
-          setError(null)
-        }
-      } catch (err) {
-        if (!cancelled) {
-          setError(err instanceof Error ? err.message : String(err))
-          setSvg(null)
-        }
-      }
-    }
-
-    void render()
-
-    return () => {
-      cancelled = true
-    }
-  }, [chart])
-
-  if (error) {
-    return (
-      <div className="my-3 rounded-md border border-border bg-muted/30 px-3 py-2 text-xs text-muted-foreground">
-        <div className="mb-1 font-medium text-foreground">Mermaid diagram error</div>
-        <pre className="whitespace-pre-wrap break-words text-[0.75rem]">{error}</pre>
-        <details className="mt-2">
-          <summary className="cursor-pointer text-muted-foreground/70 hover:text-foreground">View source</summary>
-          <pre className="mt-1 whitespace-pre-wrap break-words text-[0.75rem]">{chart}</pre>
-        </details>
-      </div>
-    )
-  }
-
-  if (!svg) {
-    return <div className="my-3 text-xs text-muted-foreground">Rendering diagram…</div>
-  }
-
-  return (
-    <div
-      ref={containerRef}
-      className="my-3 overflow-auto rounded-md border border-border bg-background p-3 [&_svg]:max-w-full"
-      dangerouslySetInnerHTML={{ __html: svg }}
-    />
-  )
-}