[SandObserver]

Summary

• Read prefers-reduced-motion in widget-toolbox.js and omit the width transition barFill sets inline when it is asked for.
• Treat a missing or throwing matchMedia as no preference, so the bar still draws.
• Note the behaviour on the barFill entry in docs/widgets.md.
• Leave sparkline unchanged: it builds a static SVG and animates nothing.

Note on scope. No shipped widget imports barFill or sparkline; every widget draws its own bars and charts, and all thirteen widget pages already carry their own prefers-reduced-motion blocks. These two helpers are documented toolkit API for the next widget to reuse, which is why this is fixed in the helper rather than left to each page.

Testing
All green, apart from the Docker build smoke test, which needs a local Docker daemon. Added 5 tests covering both preference states, the preference being read per call, and a browser with no usable matchMedia.

Checklist

• Stays within the project constraints: one container, no runtime dependencies, no build step (see CONTRIBUTING.md).
• Tests added or updated for new logic, and the full suite, lint, and typechecks pass.
• Docs updated if behavior or configuration changed.
• CHANGELOG.md updated under Unreleased if the change is user-visible.