docs(adr): Add Architecture Decision Record for dark mode theme implementation

[kike-alt]

Context

Record architectural choices for theme state management, CSS variables, and system preference detection.

Acceptance criteria

• Create docs/adr/0010-dark-mode-theme-design.md.
• Document use of CSS variables for theme tokens and localStorage persistence.
• Record accessibility rules for maintaining contrast ratios across themes.

Implementation notes

Add new ADR file under docs/adr/.