Programmatic `scrollIntoView` / `scrollTo` calls in the settings shell,
admin shell, and coach message thread ignored the user's reduced-motion
preference — a WCAG 2.3.3 violation. Add a single `scrollBehaviorForUser`
helper in `src/lib/motion.ts` that returns `"auto"` when SSR or when
`(prefers-reduced-motion: reduce)` matches, and `"smooth"` otherwise.
Wire it through the three call-sites; tests cover all three branches.
