How do you stop AI-generated code from quietly ruining your UI consistency?

I've noticed this exact same drift when building apps with AI lately.

The landing page looks gorgeous because you spent time prompting it right, but by the time you're deep into page three buttons start changing shapes, fonts look slightly off, and margins get all over the place. I tried dropping a tiny style guide into the project instructions, but the AI still slips up if a prompt gets too long.

For those building fast with AI, do you use a strict component library from the start, or just keep rewriting CSS rules as you go?