Disable web animations when the user prefers reduced motion

[LeopoldTal]

The prefers-reduced-motion media query allows users to specify an accessibility setting for less animation.

The purpose of this PR is to respect this setting in web export.

• Scrolling: Smooth scrolling triggers seasickness, so disable it. Instant scrolling is usually preferred but would be disorienting in the context of Ink games. Instead, let the user scroll at their own pace.
• Fade-in: A row of flashing paragraphs can also be a vestibular trigger. Show all the paragraphs at once.
• Colour transitions: Not necessary for seasickness reduction, but exposes current app state more clearly.

See also: PR for inkjs with similar changes.