Refactor API client and remove unused code

- Removed the ApiError class and related functions from the api-client module.
- Updated vendor detail page to handle errors without the ApiError class.
- Refactored various API modules to use in-memory mock data instead of commented-out fetch implementations.
- Removed the auth-token module as it was no longer needed.
- Updated error handling to use a more generic error structure.
- Adjusted common types to include ApiResult interface for better type management.

Frontend/erp-system/app/dashboard/procurement/purchase-orders/[id]/page.tsx

@@ -10,7 +10,6 @@ import { warehousesApi } from "@/lib/api/warehouses"
 import { itemsApi } from "@/lib/api/items"
 import { uomsApi } from "@/lib/api/uoms"
 import { vendorsApi } from "@/lib/api/vendors"
-import { ApiError } from "@/lib/api-client"
 import { errorMessage } from "@/lib/error-map"
 import { validatePoLine } from "@/lib/validations/procurement"
 import { cn } from "@/lib/utils"
@@ -172,7 +171,7 @@ export default function PurchaseOrderDetailPage() {
       setLines(toDraftLines(result.data))
       toast.success("Purchase order saved", `${result.data.docNo} updated (FR-PROC-05, edit-while-open).`)
     } catch (err) {
-      const code = err instanceof ApiError ? err.code : (err as { code?: string })?.code
+      const code = (err as { code?: string })?.code
       if (code === "CONCURRENCY_CONFLICT") {
         setConflict(true)
         setSaveError(errorMessage(err))

Frontend/erp-system/app/dashboard/products/[id]/page.tsx

@@ -10,7 +10,6 @@ import { categoriesApi } from "@/lib/api/categories"
 import { uomsApi } from "@/lib/api/uoms"
 import { vendorsApi } from "@/lib/api/vendors"
 import { warehousesApi } from "@/lib/api/warehouses"
-import { ApiError } from "@/lib/api-client"
 import { errorMessage, fieldErrors } from "@/lib/error-map"
 import { validateConversionLine, validateItemForm, validateReorderLine } from "@/lib/validations/master-data"
 import { cn } from "@/lib/utils"
@@ -147,7 +146,7 @@ export default function ItemDetailPage() {
       setEtag(result.etag)
       toast.success("Item saved", `${result.data.sku} — ${result.data.name}`)
     } catch (err) {
-      const code = err instanceof ApiError ? err.code : (err as { code?: string })?.code
+      const code = (err as { code?: string })?.code
       if (code === "CONCURRENCY_CONFLICT") {
         setConflict(true)
         setSaveError(errorMessage(err))

Frontend/erp-system/app/dashboard/vendors/[id]/page.tsx

@@ -6,7 +6,6 @@ import Link from "next/link"
 import { AlertTriangle, ArrowLeft, Save } from "lucide-react"
 
 import { vendorsApi } from "@/lib/api/vendors"
-import { ApiError } from "@/lib/api-client"
 import { errorMessage, fieldErrors } from "@/lib/error-map"
 import { cn } from "@/lib/utils"
 import { Vendor } from "@/types/master-data"
@@ -78,7 +77,7 @@ export default function VendorDetailPage() {
       setEtag(result.etag)
       toast.success("Vendor saved", `${result.data.code} — ${result.data.name}`)
     } catch (err) {
-      const code = err instanceof ApiError ? err.code : (err as { code?: string })?.code
+      const code = (err as { code?: string })?.code
       if (code === "CONCURRENCY_CONFLICT") {
         setConflict(true)
         setSaveError(errorMessage(err))