fix(desktop): isolate message render crashes from the root boundary

Streamdown runs our `preprocess` inside its own useMemo, and the user
bubble runs `extractEmbeddedImages`/directive parsing inside theirs — so
anything thrown while rendering one message (a regex/stack overflow on
adversarial content) escapes to the ROOT error boundary and takes down
the entire app, as seen in a reported `RangeError: Maximum call stack
size exceeded` from a single message.

Wrap both the assistant preprocess pipeline and the user-message
directive passes in try/catch that degrade to the raw text. One bad
message now renders plain instead of nuking the transcript.

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

@@ -327,8 +327,23 @@ function shortLabel(type: HermesRefType, id: string): string {
  * inline chips. Embedded MEDIA images render below as a thumbnail row.
  */
 export function DirectiveContent({ text }: { text: string }) {
-  const { cleanedText, images } = useMemo(() => extractEmbeddedImages(text ?? ''), [text])
-  const segments = useMemo(() => hermesDirectiveFormatter.parse(cleanedText), [cleanedText])
+  // Both passes run text through regexes; on pathological input they can throw
+  // (or overflow) and, since this renders inside a useMemo under the message,
+  // bubble up to the root error boundary. Degrade gracefully to plain text.
+  const { cleanedText, images } = useMemo(() => {
+    try {
+      return extractEmbeddedImages(text ?? '')
+    } catch {
+      return { cleanedText: text ?? '', images: [] }
+    }
+  }, [text])
+  const segments = useMemo(() => {
+    try {
+      return hermesDirectiveFormatter.parse(cleanedText)
+    } catch {
+      return [{ kind: 'text', text: cleanedText }] as Unstable_DirectiveSegment[]
+    }
+  }, [cleanedText])
 
   return (
     <span className="whitespace-pre-line" data-slot="aui_directive-text">

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

@@ -57,7 +57,16 @@ const mathPlugin = createMemoizedMathPlugin({ singleDollarTextMath: true })
 // flush) with a tail-bounded repair — see lib/remend-tail.ts. Must stay
 // module-scope so the prop identity is stable across renders.
 function preprocessWithTailRepair(text: string): string {
-  return tailBoundedRemend(preprocessMarkdown(text))
+  // Streamdown runs `preprocess` inside its own useMemo, so anything thrown
+  // here escapes to the ROOT error boundary and takes down the whole app — a
+  // single adversarial message (e.g. content that overflows a regex/stack)
+  // shouldn't be able to do that. Degrade to the raw text instead; it still
+  // renders, just without our cosmetic normalization.
+  try {
+    return tailBoundedRemend(preprocessMarkdown(text))
+  } catch {
+    return text
+  }
 }
 
 // Memoized block splitter. Streamdown calls `parseMarkdownIntoBlocks` (a full