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/NewAppointmentDrawer.tsx

@@ -309,7 +309,7 @@ export default function NewAppointmentDrawer({ doctorUuid, defaultDate, queryKey
                     <button key={s.start} type="button" dir="ltr" onClick={() => setPickedSlot({ start: s.start, end: s.end })}
                       style={{ fontSize: 13, padding: '6px 12px', borderRadius: 'var(--r-sm)', cursor: 'pointer', fontFamily: 'inherit',
                         border: active ? '1px solid var(--primary)' : '1px solid var(--border)',
-                        background: active ? 'var(--primary)' : 'var(--surface)', color: active ? '#fff' : 'var(--text)' }}>
+                        background: active ? 'var(--primary)' : 'var(--surface)', color: active ? 'var(--on-primary)' : 'var(--text)' }}>
                       {s.start_time}
                     </button>
                   );