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/ClinicServicesPage.tsx

@@ -35,7 +35,7 @@ function Avatar({ name }: { name: string }) {
       width: 26, height: 26, borderRadius: '50%',
       background: 'linear-gradient(145deg, oklch(0.62 0.15 162), oklch(0.48 0.16 162))',
       display: 'flex', alignItems: 'center', justifyContent: 'center',
-      fontSize: 11, fontWeight: 700, color: '#fff', flexShrink: 0,
+      fontSize: 11, fontWeight: 700, color: 'var(--on-primary)', flexShrink: 0,
     }}>
       {name.charAt(0)}
     </div>