fix: reduce code block highlight flicker

Preserve layout with invisible placeholders while worker highlighting loads
Reuse cached highlighted lines on first render when available
Differentiate loading from failed highlighting results for better fallback behavior

packages/ui/src/components/chat/message/parts/VirtualizedCodeBlock.tsx

@@ -54,7 +54,8 @@ export const VirtualizedCodeBlock: React.FC<VirtualizedCodeBlockProps> = React.m
   const syntaxVars = React.useMemo(() => getMarkdownSyntaxVars(currentTheme), [currentTheme]);
   // Tokenize the whole block in one worker call; rows index into the result.
   const fullText = React.useMemo(() => lines.map((line) => line.text).join('\n'), [lines]);
-  const highlighted = useWorkerHighlightedLines(fullText, language);
+  const highlightResult = useWorkerHighlightedLines(fullText, language);
+  const highlighted = highlightResult.lines;
 
   const shouldVirtualize = lines.length > VIRTUALIZE_THRESHOLD;