[A11Y] Use `prefers-reduced-motion` to calm animations

People with inner ear conditions can become nauseous and physically sick from web animations.

We should NOT remove animations, but instead of fast zooms or slides, we should favour opacity fades.

https://css-tricks.com/empathetic-animation/#did-we-allow-users-to-opt-out

- [ ] Mobile drop-down slide in
- [ ] Mobile modal slide in

Locate the mobile drop-down and mobile modal animation entry points, then inspect how their slide-in transitions are defined. Verify the prefers-reduced-motion behavior by checking that reduced-motion users receive opacity fades instead of fast slides, while normal animations remain available.

Written by the indexing model from the issue text.