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/components/layout/PurchaseSubscriptionSidebar.tsx

@@ -115,7 +115,7 @@ export default function PurchaseSubscriptionSidebar({ active }: { active: string
             padding: '0 14px', fontSize: 16, fontWeight: isActive ? 700 : 500,
             lineHeight: 1, textAlign: 'right',
             background: isActive ? 'var(--accent)' : 'transparent',
-            color: isActive ? '#FFFFFF' : 'var(--text-2)',
+            color: isActive ? 'var(--on-primary)' : 'var(--text-2)',
             cursor: item.to ? 'pointer' : 'not-allowed',
             transition: 'background .14s',
           };