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/SmsPage.tsx

@@ -382,7 +382,7 @@ export default function SmsPage() {
                   display: 'inline-flex', alignItems: 'center', justifyContent: 'center',
                   minWidth: 18, height: 18, padding: '0 4px', borderRadius: 9,
                   fontSize: 10, fontWeight: 700,
-                  background: 'oklch(0.62 0.22 30)', color: '#fff', lineHeight: 1,
+                  background: 'oklch(0.62 0.22 30)', color: 'var(--on-primary)', lineHeight: 1,
                 }}>
                   {t.badge}
                 </span>