fix: Add semantic CSS custom property aliases for theming

[williamzujkowski]

Found during: UX gap analysis

Colors use raw Tailwind theme values but lack semantic aliases like --color-text-primary, --color-surface, --color-text-muted. Semantic tokens improve maintainability and make dark/light mode switching more systematic.

Proposed tokens:

• --color-text-primary / --color-text-secondary / --color-text-muted
• --color-surface / --color-surface-elevated
• --color-border / --color-border-muted
• --color-diff-add / --color-diff-remove
• --color-accent / --color-accent-muted