Performance: Memoize chat message rendering to reduce markdown rerenders #29

Problem

Chat message list rerenders full history and reparses markdown for every update.

Scope

• Memoize message row rendering and minimize markdown reprocessing.
• Keep current visual output.

Acceptance Criteria

• Lower rerender cost for long conversations.
• Message rendering behavior remains unchanged.

References

• components/ai-chat/chat-messages.tsx