feat: Phase 7.2 — wire highlightTerms into ResponsePanel + mark CSS

- Add HighlightMark component rendering <mark class="bg-yellow-200...">
- Call highlightTerms() in SubQuestionSection and FlatResponse before ReactMarkdown
- Add mark: HighlightMark to ReactMarkdown components in both paths
- Add .prose mark CSS rule (yellow-200 bg, rounded, px-0.5)
- Tests: 56/56 pass (citation + highlight + ResponsePanel)

frontend/src/components/ResponsePanel.tsx

@@ -3,7 +3,7 @@ import { MessageSquare, AlertCircle, Copy, ChevronDown, ChevronRight } from 'luc
 import ReactMarkdown from 'react-markdown'
 import type { SourceMetadata, SubQuestionSources } from '../types'
 import { getPdfViewerUrl } from '../lib/api'
-import { processCitations, processCitationsForSubq, extractCitedSources } from '../utils/citationParser'
+import { processCitations, processCitationsForSubq, extractCitedSources, highlightTerms } from '../utils/citationParser'
 import { bulletizeMarkdown } from '../utils/citationParser'
 
 function getHighlightUrl(document_id: string, chunk_index: number, sub_question: string): string {
@@ -32,6 +32,10 @@ const CitationLink = ({ href, children }: { href?: string; children?: React.Reac
   </a>
 )
 
+const HighlightMark = ({ children }: { children?: React.ReactNode }) => (
+  <mark className="bg-yellow-200 rounded px-0.5">{children}</mark>
+)
+
 function parseAnswerSections(answer: string): string[] {
   const sections = answer.split(/## Sub-question \d+:[^\n]*\n/)
   if (sections.length <= 1) return [bulletizeMarkdown(answer)]
@@ -107,6 +111,7 @@ function SubQuestionSection({
     sq.sources.map(s => ({ ...s, sub_question_text: sq.sub_question_text }))
   )
   const processedAnswer = processCitations(answerSection, allSources, highlightReadyKeys)
+  const highlightedAnswer = highlightTerms(processedAnswer)
 
   return (
     <div
@@ -120,8 +125,8 @@ function SubQuestionSection({
       </div>
 
       <div className="prose prose-sm max-w-none text-gray-800">
-        <ReactMarkdown components={{ a: CitationLink }}>
-          {processedAnswer}
+        <ReactMarkdown components={{ a: CitationLink, mark: HighlightMark }}>
+          {highlightedAnswer}
         </ReactMarkdown>
       </div>
 
@@ -353,6 +358,7 @@ function FlatResponse({
   const safeSources = sources ?? []
 
   const processedAnswer = answer ? processCitations(answer, safeSources) : answer ?? ''
+  const highlightedAnswer = processedAnswer ? highlightTerms(processedAnswer) : processedAnswer
 
   const handleCopyAnswer = async (): Promise<void> => {
     if (answer) {
@@ -454,9 +460,9 @@ function FlatResponse({
         </div>
         <div className="prose prose-sm max-w-none text-gray-800">
           <ReactMarkdown
-            components={{ a: CitationLink }}
+            components={{ a: CitationLink, mark: HighlightMark }}
           >
-            {processedAnswer}
+            {highlightedAnswer}
           </ReactMarkdown>
         </div>
       </div>

frontend/src/styles.css

@@ -10,3 +10,8 @@
   list-style: decimal !important;
   padding-left: 1.5rem !important;
 }
+.prose mark {
+  background-color: #FEF08A;
+  border-radius: 0.125rem;
+  padding: 0 0.125rem;
+}