feat: add highlight status toast notification (Phase 5.4)

- Shows 'Preparing highlights...' (amber spinner) while LLM batch runs
- Shows 'Highlights ready' (green) for 4 seconds when batch completes
- Fixed position top-left corner, auto-dismisses

frontend/src/components/ResponsePanel.tsx

@@ -167,6 +167,7 @@ function SubQuestionSections({
 }) {
   const [copied, setCopied] = useState(false)
   const [highlightReadyKeys, setHighlightReadyKeys] = useState<Set<string>>(new Set())
+  const [highlightStatus, setHighlightStatus] = useState<'loading' | 'done' | ''>('')
 
   useEffect(() => {
     if (!answer || isLoading || !subQuestionSources.length) return
@@ -197,6 +198,8 @@ function SubQuestionSections({
     
     if (targets.length === 0) return
     
+    setHighlightStatus('loading')
+    
     fetch('/api/v1/v2/highlights/batch', {
       method: 'POST',
       headers: { 'Content-Type': 'application/json' },
@@ -210,6 +213,8 @@ function SubQuestionSections({
             keys.add(`${t.document_id}_${t.chunk_index}_${encodeURIComponent(t.sub_question_text)}`)
           })
           setHighlightReadyKeys(keys)
+          setHighlightStatus('done')
+          setTimeout(() => setHighlightStatus(''), 4000)
         }
       })
       .catch((err) => {
@@ -276,6 +281,24 @@ function SubQuestionSections({
 
   return (
     <div className="p-4 space-y-4">
+      {highlightStatus !== '' && (
+        <div
+          data-testid="highlight-status-toast"
+          className={`fixed top-4 left-4 z-50 px-4 py-2 rounded-lg shadow-lg text-sm font-medium transition-opacity duration-300 ${
+            highlightStatus === 'done'
+              ? 'bg-green-50 border border-green-200 text-green-700'
+              : 'bg-amber-50 border border-amber-200 text-amber-700'
+          }`}
+        >
+          {highlightStatus === 'loading' && (
+            <span className="flex items-center gap-2">
+              <span className="w-3 h-3 border-2 border-amber-400 border-t-transparent rounded-full animate-spin" />
+              Preparing highlights...
+            </span>
+          )}
+          {highlightStatus === 'done' && 'Highlights ready'}
+        </div>
+      )}
       <div className="flex items-start justify-between">
         <div className="flex-1" />
         <button