Implement OAuth callback support for MCP servers

- Add react-router-dom dependency for routing support
- Create OAuthCallback component with animated background and loading spinner
- Add routing to App.tsx with /oauth/callback route
- Match styling with main app using animated-bg-container
- Use onMcpAuthorization from use-mcp library for OAuth handling
- Library already handles multiple server differentiation via state parameter
- Each MCP server gets unique OAuth flow identified by state containing server URL
- Clean up unused imports and variables in McpServerModal

examples/chat-ui/package.json

@@ -26,6 +26,7 @@
     "react": "^19.0.0",
     "react-dom": "^19.0.0",
     "react-markdown": "^10.1.0",
+    "react-router-dom": "^7.6.2",
     "rehype-highlight": "^7.0.2",
     "remark-gfm": "^4.0.1",
     "tailwindcss": "^4.0.14",