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/TenantInsuranceContracts.tsx

@@ -382,7 +382,7 @@ function StatusToggle({ contract, onToggle, disabled }: { contract: Contract; on
         background: contract.is_active ? 'var(--primary)' : 'var(--border)',
       }}>
         <span style={{
-          position: 'absolute', top: 2, width: 16, height: 16, borderRadius: 999, background: '#fff', transition: 'inset-inline .2s',
+          position: 'absolute', top: 2, width: 16, height: 16, borderRadius: 999, background: 'var(--surface)', transition: 'inset-inline .2s',
           insetInlineStart: contract.is_active ? 18 : 2,
         }} />
       </span>