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/layout/Topbar.tsx

@@ -122,7 +122,7 @@ export default function Topbar({ onMobileMenuOpen }: { onMobileMenuOpen?: () =>
                         width: 40, height: 40, borderRadius: 10,
                         background: hue.fixed?.primary ?? `oklch(0.55 ${hue.c} ${hue.h})`,
                         border: brandHue.h === hue.h ? '3px solid var(--text)' : '3px solid transparent',
-                        display: 'grid', placeItems: 'center', color: '#fff', cursor: 'pointer',
+                        display: 'grid', placeItems: 'center', color: 'var(--on-primary)', cursor: 'pointer',
                       }}
                     >
                       {brandHue.h === hue.h && <CheckIcon style={{ width: 16, height: 16 }} />}