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/SelectContextPage.tsx

@@ -57,7 +57,7 @@ export default function SelectContextPage() {
                   padding: '.875rem 1rem',
                   border: '1.5px solid var(--border, #e2e8f0)',
                   borderRadius: '0.625rem',
-                  background: loading === ctx.db_uuid ? 'var(--surface-alt, #f4f6f9)' : '#fff',
+                  background: loading === ctx.db_uuid ? 'var(--surface-alt, #f4f6f9)' : 'var(--surface)',
                   cursor: loading !== null ? 'wait' : 'pointer',
                   textAlign: 'right',
                   transition: 'border-color .15s, box-shadow .15s',