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/ui/StatCard.tsx

@@ -33,7 +33,7 @@ export default function StatCard({ tone, label, value, icon }: Props) {
         <div
           style={{
             width: 44, height: 44, borderRadius: 12, flexShrink: 0,
-            background: c.fg, color: '#fff',
+            background: c.fg, color: 'var(--on-primary)',
             display: 'flex', alignItems: 'center', justifyContent: 'center',
           }}
         >