feat(frontend): Add customers API client

Created customers.ts API client following coupons pattern

Types:
✅ Customer - Full customer data
✅ CustomerAddress - Billing/shipping address
✅ CustomerStats - Order statistics
✅ CustomerListResponse - Paginated list response
✅ CustomerFormData - Create/update payload
✅ CustomerSearchResult - Autocomplete result

API Methods:
✅ list() - Get customers with pagination/search/filter
✅ get() - Get single customer with full details
✅ create() - Create new customer
✅ update() - Update customer data
✅ delete() - Delete customer
✅ search() - Autocomplete search

Next: Create CRUD pages (index, new, edit)

admin-spa/src/lib/api/customers.ts

@@ -0,0 +1,109 @@
+import { api } from '../api';
+
+export interface CustomerAddress {
+  first_name: string;
+  last_name: string;
+  company?: string;
+  address_1: string;
+  address_2?: string;
+  city: string;
+  state?: string;
+  postcode: string;
+  country: string;
+  phone?: string;
+}
+
+export interface CustomerStats {
+  total_orders: number;
+  total_spent: number;
+}
+
+export interface Customer {
+  id: number;
+  username: string;
+  email: string;
+  first_name: string;
+  last_name: string;
+  display_name: string;
+  registered: string;
+  role: string;
+  billing?: CustomerAddress;
+  shipping?: CustomerAddress;
+  stats?: CustomerStats;
+}
+
+export interface CustomerListResponse {
+  data: Customer[];
+  pagination: {
+    total: number;
+    total_pages: number;
+    current: number;
+    per_page: number;
+  };
+}
+
+export interface CustomerFormData {
+  email: string;
+  first_name: string;
+  last_name: string;
+  username?: string;
+  password?: string;
+  billing?: Partial<CustomerAddress>;
+  shipping?: Partial<CustomerAddress>;
+  send_email?: boolean;
+}
+
+export interface CustomerSearchResult {
+  id: number;
+  name: string;
+  email: string;
+}
+
+export const CustomersApi = {
+  /**
+   * List customers with pagination and filtering
+   */
+  list: async (params?: {
+    page?: number;
+    per_page?: number;
+    search?: string;
+    role?: string;
+  }): Promise<CustomerListResponse> => {
+    return api.get('/customers', { params });
+  },
+
+  /**
+   * Get single customer
+   */
+  get: async (id: number): Promise<Customer> => {
+    return api.get(`/customers/${id}`);
+  },
+
+  /**
+   * Create new customer
+   */
+  create: async (data: CustomerFormData): Promise<Customer> => {
+    return api.post('/customers', data);
+  },
+
+  /**
+   * Update customer
+   */
+  update: async (id: number, data: Partial<CustomerFormData>): Promise<Customer> => {
+    return api.put(`/customers/${id}`, data);
+  },
+
+  /**
+   * Delete customer
+   */
+  delete: async (id: number): Promise<void> => {
+    return api.del(`/customers/${id}`);
+  },
+
+  /**
+   * Search customers (for autocomplete)
+   */
+  search: async (query: string, limit?: number): Promise<CustomerSearchResult[]> => {
+    return api.get('/customers/search', { params: { q: query, limit } });
+  },
+};