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/appointmentStatusMeta.test.ts

@@ -23,7 +23,7 @@ describe('STATUS_META', () => {
   it.each(BACKEND_STATUSES)('has a Persian label and colour for %s', (status) => {
     expect(STATUS_META[status]).toBeDefined();
     expect(STATUS_META[status].label.trim()).not.toBe('');
-    expect(STATUS_META[status].color).toMatch(/^#[0-9a-f]{6}$/i);
+    expect(STATUS_META[status].color).toMatch(/^var\(--[a-z0-9-]+\)$/);
   });
 
   it('carries no label that is still English', () => {