Motivation

• Ensure PUI honors OS/browser accessibility preferences by disabling or reducing non-essential motion and preserving critical affordances in high-contrast environments.
• Provide predictable, CSS-first low-motion fallbacks for the most common animated primitives so overlays and loaders remain usable without JS feature flags.
• Document the platform media-query behavior so theme authors and app integrators follow the same declarative approach.
• Add a visual demo route so maintainers can quickly validate reduced-motion and forced-colors behavior during development.

Description

• Added accessibility media-query baselines to src/base.css with @media (prefers-reduced-motion: reduce) to suppress animations/transitions and @media (forced-colors: active) to expose system focus/border semantics.
• Added targeted low-motion fallbacks in component styles for animated primitives: src/components/dialog/style.css, src/components/drawer/style.css, src/components/sheet/style.css, src/components/skeleton/style.css, src/components/spinner/style.css, and src/lib/toast.css (reduced-motion + forced-colors adjustments).
• Added a demo route demo/src/routes/a11y-modes.tsx, wired it into the router (demo/src/main.tsx) and navigation (demo/src/nav.tsx), and included demo-specific styles in demo/src/style.css to surface sanity checks.
• Updated documentation with an “Accessibility Media Queries” section and high-contrast guidance in docs/pages/base-styles.md and docs/pages/theming.md describing recommended usage and sample CSS.

Testing

• Ran pnpm build at the repo root and the build completed successfully.
• Ran pnpm build in the demo/ directory and the demo built successfully and produced the demo assets.
• Started the demo and captured an automated screenshot of /a11y-modes using Playwright to validate visuals (artifact produced).
• Ran pnpm lint which failed due to pre-existing TypeScript issues unrelated to these accessibility CSS and demo changes (no failing tests attributable to this PR).

Codex Task