docs: update codebase best practices

[majestic-owl448]

Checklist:

• I have read freeCodeCamp's contribution guidelines.
• My pull request has a descriptive title (not a vague title like Update index.md)

• Clarify how the client imports @freecodecamp/ui styles and how to preserve file history during JavaScript-to-TypeScript migrations.
• Correct the Redux examples so they contain valid TypeScript and consistent payload handling.
• Update the API testing guidance for the current Vitest and Fastify test structure.