feat: add light theme and configurable accent color #37

Summary

Add a light theme (the spec requires dark graphite + light) and let users pick an accent color.

Motivation

Accessibility and preference; bright rooms and some handheld screens favor light UIs.

Requirements

• Theme tokens for both themes; follow Windows setting by default.
• Accent color picker usable with the controller; contrast checked.

Controller UX

Menu → Appearance → Theme / Accent.

Technical Notes

Extend Theme tokens; persist in settings.

Acceptance Criteria

• Both themes meet contrast targets; accent changes apply live.

Testing

Manual visual review; screenshot in smoke.

Dependencies

• None

Priority: medium · Milestone: 2 · Controller-First Windows UI