Implement comprehensive dark/light mode overhaul for admin panel

- Refactor color palette in `ui-design-spec.md` to utilize CSS variables exclusively, eliminating fixed hex values and Tailwind utility classes.
- Complete dark mode implementation in `uiStore.ts`, ensuring proper theme application via `applyTheme()` and `applyBrand()`.
- Create `admin-theme-dark-light-audit.md` to document the transition process, outlining issues with inline styles and fixed colors.
- Introduce `theme-tokens.test.ts` to enforce rules against fixed hex colors and ensure compliance with the design system.
- Update various components and styles to replace inline styles and fixed colors with CSS variables, ensuring consistent theming across light and dark modes.
- Ensure all changes maintain visual integrity in both light and dark modes, with a focus on accessibility and contrast standards.

assets/admin/pages/AppointmentsPage.tsx

@@ -75,7 +75,7 @@ function DateNavigator({ date, onChange }: { date: string; onChange: (d: string)
         background: 'var(--surface)', border: '1px solid var(--border)',
         borderRadius: 'var(--r-sm)', minWidth: 130, gap: 1,
       }}>
-        <span style={{ fontSize: 13, fontWeight: 700, lineHeight: 1.2, color: isFriday ? '#ef4444' : 'var(--text)' }}>
+        <span style={{ fontSize: 13, fontWeight: 700, lineHeight: 1.2, color: isFriday ? 'var(--danger)' : 'var(--text)' }}>
           {weekDay}
         </span>
         <span style={{ fontSize: 11, color: 'var(--text-2)', lineHeight: 1.2 }}>