feat(admin): design system audit fixes

- Add CSS design tokens for consistency (--admin-danger-hover, --admin-accent-dot, shadow-md warm tone)
- Replace unicode icons with inline SVG (⋮, ✕, ▼, ▶)
- Consolidate duplicate PageHeader/AdminPageHeader components
- Standardize border-radius (rounded-xl → rounded-lg for ViewModeTabs)
- Remove hardcoded brand UUID in products page
- Replace hardcoded bg-red-600 with CSS variables in delete buttons
- Add AdminButton, AdminFilterTabs, AdminSearchInput, PageHeader design system components
- Fix GlassModal and AdminModal close button styling
- Remove duplicate 'New Lot' button on Route Trace dashboard

src/components/admin/AdminHeader.tsx

@@ -1,3 +1,8 @@
+/**
+ * @deprecated Use AdminSidebar instead. AdminHeader is deprecated and will be removed.
+ * The AdminSidebar provides the complete navigation system for admin pages.
+ * It includes an expandable Settings sub-menu with all settings pages.
+ */
 "use client";
 
 import Link from "next/link";