Update CODING_STANDARDS.md

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

CODING_STANDARDS.md

@@ -75,6 +75,8 @@ Remember, these rules are here to make our codebase harmonious. If something doe
   - Avoid: `import * as React from "react";` then `React.useCallback(...)`
 - Add `data-testid` to testable elements for Playwright
 - Co-locate utilities that are truly component-specific next to the component, otherwise place shared items under a common folder
+- Avoid mixed controlled and uncontrolled state in React components. A component is either controlled or uncontrolled. State needs a single source of truth instead of being computed by props and then recomputed internally.
+- SHOULD: Use derived state variables instead of adding unneeded `React.useState` / `useState` hooks.
 
 
 ## Readability and Abstractions