Step 4: Add admin UI page for pricing configuration

- Add pricing API functions to admin.ts
- Create admin pricing page with form and validation
- Add MANAGE_PRICING permission to auth context
- Add pricing to admin navigation
- Add translations for pricing page (en, ca, es)
- Update PageLayout and Header types for new page

frontend/app/components/Header.tsx

@@ -17,7 +17,8 @@ type PageId =
   | "admin-invites"
   | "admin-availability"
   | "admin-trades"
-  | "admin-price-history";
+  | "admin-price-history"
+  | "admin-pricing";
 
 interface HeaderProps {
   currentPage: PageId;
@@ -48,6 +49,7 @@ const ADMIN_NAV_ITEMS: NavItem[] = [
   },
   { id: "admin-invites", labelKey: "invites", href: "/admin/invites", adminOnly: true },
   { id: "admin-price-history", labelKey: "prices", href: "/admin/price-history", adminOnly: true },
+  { id: "admin-pricing", labelKey: "pricing", href: "/admin/pricing", adminOnly: true },
 ];
 
 export function Header({ currentPage }: HeaderProps) {