a11y/motion: formalize reduced-motion support

[ArchILLtect]

Summary

Formalize reduced-motion support

Motivation / Problem

Description:

• Ensure route transitions and health status animations are disabled or minimized when prefers-reduced-motion: reduce is set.
• Provide a dev toggle to test reduced motion locally.
• Ensure .env.example and README enumerate all supported VITE_* vars.
• Add a tiny runtime guard that warns (or throws in dev) when a required variable is missing or invalid.

Acceptance Criteria

Acceptance Criteria:

• Visual inspection + one simple test asserting motion-disabled path is taken.

Target Release

0.2.0

Notes / Links

No response