feat(desktop): render embeds, fences and alerts in assistant markdown

Wire the embeds module into the markdown surface: bare provider autolinks unfurl
to inline embeds, ```mermaid/```svg fences route to the rich renderers, and
`> [!NOTE]`-style blockquotes become alert callouts. Labeled links stay plain.

apps/desktop/src/components/assistant-ui/embeds/index.ts

@@ -0,0 +1,5 @@
+export { extractAlert, MarkdownAlert } from './alert'
+export type { EmbedDescriptor } from './providers'
+export { detectEmbed, isEmbeddableUrl } from './providers'
+export { RICH_FENCE_LANGUAGES, RichCodeBlock } from './registry'
+export { UrlEmbed } from './url-embed'

apps/desktop/src/components/assistant-ui/markdown-text.tsx

@@ -40,6 +40,8 @@ import { previewTargetFromMarkdownHref } from '@/lib/preview-targets'
 import { tailBoundedRemend } from '@/lib/remend-tail'
 import { cn } from '@/lib/utils'
 
+import { detectEmbed, extractAlert, MarkdownAlert, RichCodeBlock, UrlEmbed } from './embeds'
+
 // Math rendering plugin (KaTeX). Configured once at module scope — the
 // plugin is stateless beyond its internal cache so re-creating per-render
 // would needlessly thrash. We use a memoizing wrapper around rehype-katex
@@ -270,6 +272,17 @@ function MarkdownLink({ children, className, href, ...props }: ComponentProps<'a
   }
 
   const text = childrenToText(children)
+
+  // Bare autolink → inline rich embed when a provider matches. Labeled links
+  // (`[watch](url)`) stay plain. Desktop only (webview / iframe renderers).
+  if (window.hermesDesktop && text && normalizeExternalUrl(text) === target) {
+    const embed = detectEmbed(target)
+
+    if (embed) {
+      return <UrlEmbed descriptor={embed} />
+    }
+  }
+
   const fallbackLabel = text && normalizeExternalUrl(text) !== target ? text : undefined
 
   return (
@@ -535,13 +548,25 @@ function MarkdownTextSurface({ containerClassName, containerProps }: MarkdownTex
         // owning per-line text direction. Inline code carries `dir="ltr"`
         // (see the `code` override) so it doesn't vote here either, same
         // contract as the CSS isolate.
-        blockquote: ({ className, ...props }: ComponentProps<'blockquote'>) => (
-          <blockquote
-            className={cn('border-s-2 border-border ps-3 text-muted-foreground italic', className)}
-            dir="auto"
-            {...props}
-          />
-        ),
+        // A `> [!NOTE]`/`[!WARNING]`/... blockquote renders as a GFM alert
+        // callout; everything else stays a plain quote.
+        blockquote: ({ children, className, ...props }: ComponentProps<'blockquote'>) => {
+          const alert = extractAlert(children)
+
+          if (alert) {
+            return <MarkdownAlert type={alert.type}>{alert.body}</MarkdownAlert>
+          }
+
+          return (
+            <blockquote
+              className={cn('border-s-2 border-border ps-3 text-muted-foreground italic', className)}
+              dir="auto"
+              {...props}
+            >
+              {children}
+            </blockquote>
+          )
+        },
         ul: ({ className, ...props }: ComponentProps<'ul'>) => (
           <ul className={cn('my-1 gap-0', className)} dir="auto" {...props} />
         ),
@@ -578,7 +603,16 @@ function MarkdownTextSurface({ containerClassName, containerProps }: MarkdownTex
           <td className={cn('px-2.5 py-1.5 align-top text-[0.8125rem] leading-snug', className)} {...props} />
         ),
         img: MarkdownImage,
-        SyntaxHighlighter: (props: SyntaxHighlighterProps) => <SyntaxHighlighter {...props} defer={isStreaming} />
+        // ```mermaid / ```svg fences route to their lazy renderers; every other
+        // language falls back to the Shiki-highlighted code block.
+        SyntaxHighlighter: (props: SyntaxHighlighterProps) => (
+          <RichCodeBlock
+            code={props.code}
+            fallback={<SyntaxHighlighter {...props} defer={isStreaming} />}
+            language={props.language}
+            streaming={isStreaming}
+          />
+        )
       }) as StreamdownTextComponents,
     [isStreaming]
   )