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/CategoriesPage.tsx

@@ -90,7 +90,7 @@ function LogoUploadField({ value, onChange, uploadUrl }: {
           <img src={value} alt="logo" style={{ width: 64, height: 64, objectFit: 'contain', borderRadius: 12, border: '1px solid var(--border)', background: 'var(--surface)', padding: 4 }} />
           <button type="button" onClick={() => onChange(null)} style={{
             position: 'absolute', top: -6, right: -6, width: 20, height: 20, borderRadius: 10,
-            background: 'var(--danger)', color: '#fff', border: 'none', cursor: 'pointer',
+            background: 'var(--danger)', color: 'var(--on-primary)', border: 'none', cursor: 'pointer',
             display: 'flex', alignItems: 'center', justifyContent: 'center',
           }}>
             <XMarkIcon style={{ width: 12, height: 12 }} />