Add React.memo to ChunkCard in source detail panel

[MODSetter]

Description

ChunkCard in source-detail-panel.tsx renders a MarkdownViewer (which uses Streamdown + KaTeX) for each document chunk. When activeChunkIndex or scroll state changes, all chunk cards re-render even though their markdown content hasn't changed. Since MarkdownViewer is expensive, memoizing ChunkCard can significantly reduce wasted work.

Vercel React Best Practices Rule: rerender-memo (5.5)

File to change

• surfsense_web/components/new-chat/source-detail-panel.tsx

What to do

1. Add memo to the React import (alongside the existing forwardRef).

2. Wrap the existing forwardRef component in memo (lines 58-105):

// Before
const ChunkCard = forwardRef<HTMLDivElement, ChunkCardProps>(
  ({ chunk, index, totalChunks, isCited }, ref) => {
    return ( ... );
  }
);

// After
const ChunkCard = memo(forwardRef<HTMLDivElement, ChunkCardProps>(
  ({ chunk, index, totalChunks, isCited }, ref) => {
    return ( ... );
  }
));

Acceptance criteria

• ChunkCard is wrapped in memo
• Cited chunk highlighting still works
• Scrolling to cited chunk still works
• Non-active chunks don't re-render when active chunk changes (verifiable in React DevTools Profiler)