v035 — Polish + accessibility (closes UI track)

Final milestone of the v026-v035 UI modernization track. New: WCAG AA contrast tests covering every chrome color pair (INK on CANVAS, primary CTA on/off, error text, dark mode), bumping ERROR_TEXT to Tailwind red-700 to clear 4.5:1 on white. New: prefers_reduced_motion(ctx) and effective(ctx, duration) helpers detecting egui's animation_time == 0 signal plus BSE_REDUCE_MOTION env var. With this BSE ships ~80-85% of the Miro/Linear look without leaving the egui stack.