fix: show reduced-motion banner with opt-in override (fixes #89)

[FrancoKaddour]

When prefers-reduced-motion: reduce is active, the CSS in Layout.astro correctly disables all animations — but the page gives no feedback. Cards still show the "Hover to preview" label, pointing to an interaction that silently does nothing. That's the confusion reported in #89.

This adds ReducedMotionBanner.astro, a small dismissible banner that:

1. Detects the media query on load and shows a plain explanatory message
2. Offers an opt-in button that re-enables animations for the session via data-force-motion on , then dispatches a motionrestored event so cards can re-bind their listeners
3. Listens for runtime changes to the OS setting

The CSS override is scoped to [data-force-motion="true"] with revert so it only undoes what the existing reduced-motion rule applied — nothing else changes.

The pattern follows WCAG 2.5.3: respect the preference by default, allow explicit opt-in.

Files changed

• web/src/components/ui/ReducedMotionBanner.astro — new component
• web/src/layouts/Layout.astro — adds [data-force-motion] CSS rule
• web/src/pages/index.astro — renders the banner above the animation grid

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