feat: add warehouse and bin management API with mock data implementation

- Implemented warehouses API with methods for listing, creating, and managing bins.
- Added wastage API to handle stock write-offs and integrate with stock adjustments.
- Created auth token management for storing and retrieving access tokens.
- Developed error mapping for consistent user-facing error messages.
- Introduced client-side validations for GRN, master data, and procurement processes.
- Defined common types for pagination, problem details, and various master data entities.
- Established procurement and stock management types to support frontend functionality.

Frontend/erp-system/lib/auth-token.ts

@@ -0,0 +1,16 @@
+// Bearer-token storage for the ERPCore API client (docs/20-FRONTEND.md §1).
+// Login (POST /auth/login) is not wired yet — reads simply return null until it is,
+// and the API client omits the Authorization header in that case.
+
+const STORAGE_KEY = "erpcore.accessToken"
+
+export function getAccessToken(): string | null {
+  if (typeof window === "undefined") return null
+  return window.localStorage.getItem(STORAGE_KEY)
+}
+
+export function setAccessToken(token: string | null) {
+  if (typeof window === "undefined") return
+  if (token) window.localStorage.setItem(STORAGE_KEY, token)
+  else window.localStorage.removeItem(STORAGE_KEY)
+}