add reduceMotion to ClientRouter props

[DerTimonius]

Body

Summary

Add a reduceMotion prop to ClientRouter to allow programmatic control of view transitions, enabling developers to build accessibility toolbars that let users disable animations on a per-site basis.

Background & Motivation

Astro's ClientRouter respects the system-level prefers-reduced-motion setting, but some users may want granular control over animations on specific websites without changing system settings.

Use case: Accessibility toolbars that offer per-site preferences like font size, dyslexia-friendly fonts, and motion reduction. Users can discover and toggle these preferences without knowing about system settings.

Goals

• Give users the option to opt out of view transitions without fiddling with system settings
• Programmatically change whether or not view transitions should be shown

Example

<ClientRouter reduceMotion={userPreference} />

Behavior

• undefined (default): Respect system prefers-reduced-motion
• true: Disable view transitions regardless of system setting
• false: Enable view transitions regardless of system setting

Alternative approach

Expose a setReducedMotion API from "astro:transitions" to achieve a similar outcome