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/inventory/InventoryItemsTable.tsx

@@ -24,7 +24,7 @@ export default function InventoryItemsTable({ items, onEdit, onDelete }: Props)
       {/* Desktop table */}
       <div
         className="inv-desktop"
-        style={{ border: '1px solid #E7E7E7', borderRadius: 8, overflow: 'hidden' }}
+        style={{ border: '1px solid var(--border)', borderRadius: 8, overflow: 'hidden' }}
       >
         <table className="inv-table">
           <thead>