feat: render Markdown in AI chat responses (bold, lists, code)

Added react-markdown to render assistant messages with proper formatting:
- **bold** text renders as bold
- Bullet/numbered lists render correctly
- Inline code renders with monospace background
- User messages stay as plain text

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/package.json

@@ -24,6 +24,7 @@
     "react": "^18.3.1",
     "react-dom": "^18.3.1",
     "react-dropzone": "^14.2.3",
+    "react-markdown": "^10.1.0",
     "react-router-dom": "^6.22.3",
     "recharts": "^3.7.0",
     "sonner": "^1.4.41",