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/MyPaymentsPage.tsx

@@ -47,7 +47,7 @@ function Avatar({ name }: { name: string | null }) {
     <div style={{
       width: 32, height: 32, borderRadius: '50%', flexShrink: 0,
       background: 'linear-gradient(145deg, var(--primary), var(--primary-700, var(--primary)))',
-      display: 'grid', placeItems: 'center', color: '#fff', fontSize: 13, fontWeight: 700,
+      display: 'grid', placeItems: 'center', color: 'var(--on-primary)', fontSize: 13, fontWeight: 700,
     }}>
       {(name ?? '؟').charAt(0)}
     </div>