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/api-client.ts

@@ -0,0 +1,98 @@
+// Single typed fetch client for the ERPCore API (docs/20-FRONTEND.md §1: "A single
+// typed fetch client against NEXT_PUBLIC_API_BASE_URL; all calls go through it. No
+// scattered fetch() in components."). Per-endpoint methods live in lib/api/*.ts.
+import { ProblemDetails } from "@/types/common"
+import { getAccessToken } from "@/lib/auth-token"
+
+const API_BASE = `${process.env.NEXT_PUBLIC_API_BASE_URL ?? ""}/api/v1`
+
+/** Normalized RFC 7807 error (docs/11-BACKEND-PHASE1.md §1.8) thrown on any non-2xx response. */
+export class ApiError extends Error {
+  status: number
+  code?: string
+  detail?: string
+  errors?: Record<string, string[]>
+  traceId?: string
+
+  constructor(problem: ProblemDetails) {
+    super(problem.title || "Request failed")
+    this.status = problem.status
+    this.code = problem.code
+    this.detail = problem.detail
+    this.errors = problem.errors
+    this.traceId = problem.traceId
+  }
+}
+
+export interface RequestOptions extends Omit<RequestInit, "body"> {
+  body?: unknown
+  /** Sent as If-Match for concurrency-guarded PUT/PATCH (docs/11 §1.6). */
+  ifMatch?: string
+  /** Sent as Idempotency-Key for transactional POSTs (e.g. GRN confirm, docs/11 §1.6). */
+  idempotencyKey?: string
+}
+
+export interface ApiResult<T> {
+  data: T
+  /** Response ETag header, when the resource is mutable (docs/11 §1.6). */
+  etag: string | null
+}
+
+async function rawRequest(path: string, options: RequestOptions = {}): Promise<Response> {
+  const { body, ifMatch, idempotencyKey, headers, ...rest } = options
+  const token = getAccessToken()
+
+  const finalHeaders: Record<string, string> = {
+    Accept: "application/json",
+    ...(body !== undefined ? { "Content-Type": "application/json" } : {}),
+    ...(token ? { Authorization: `Bearer ${token}` } : {}),
+    ...(ifMatch ? { "If-Match": ifMatch } : {}),
+    ...(idempotencyKey ? { "Idempotency-Key": idempotencyKey } : {}),
+    ...((headers as Record<string, string> | undefined) ?? {}),
+  }
+
+  const response = await fetch(`${API_BASE}${path}`, {
+    ...rest,
+    headers: finalHeaders,
+    body: body !== undefined ? JSON.stringify(body) : undefined,
+  })
+
+  if (!response.ok) {
+    let problem: ProblemDetails
+    try {
+      problem = (await response.json()) as ProblemDetails
+    } catch {
+      problem = { title: response.statusText || "Request failed", status: response.status }
+    }
+    if (!problem.status) problem.status = response.status
+    throw new ApiError(problem)
+  }
+
+  return response
+}
+
+/** Fire a request and decode the JSON body only (no ETag needed). */
+export async function apiRequest<T>(path: string, options?: RequestOptions): Promise<T> {
+  const response = await rawRequest(path, options)
+  if (response.status === 204) return undefined as T
+  return (await response.json()) as T
+}
+
+/** Fire a request and also surface the ETag header, for resources that support If-Match. */
+export async function apiRequestWithETag<T>(path: string, options?: RequestOptions): Promise<ApiResult<T>> {
+  const response = await rawRequest(path, options)
+  const etag = response.headers.get("ETag")
+  const data = response.status === 204 ? (undefined as T) : ((await response.json()) as T)
+  return { data, etag }
+}
+
+/** Build a `?a=1&b=2` query string, dropping null/undefined/empty values. */
+export function buildQuery(params: object): string {
+  const search = new URLSearchParams()
+  for (const [key, value] of Object.entries(params) as [string, string | number | boolean | null | undefined][]) {
+    if (value === null || value === undefined || value === "") continue
+    search.set(key, String(value))
+  }
+  const qs = search.toString()
+  return qs ? `?${qs}` : ""
+}