feat: initialize React admin build pipeline (F2.1-F2.6)

- Add package.json with @wordpress/scripts and React dependencies
- Configure webpack for admin bundle output
- Create src/admin directory structure (api, components, pages)
- Implement API client with nonce handling (ajaxRequest, apiRequest)
- Add API methods for orders, customers, products, forms, coupons, licenses
- Create React App component with page routing
- Add placeholder page components for all admin sections
- Create ReactAdmin PHP class to manage asset enqueuing
- Register ReactAdmin singleton in main plugin file
- Bump version to 2.0.0

Build: Run 'npm install && npm run build' to generate assets

src/admin/api/client.js

@@ -0,0 +1,128 @@
+/**
+ * API client for Formipay admin interface
+ * Handles nonce-secured requests to WordPress admin-ajax.php
+ */
+
+export const API_BASE = window.formipayAdmin?.ajaxUrl || '/wp-admin/admin-ajax.php';
+export const NONCE = window.formipayAdmin?.nonce || '';
+export const REST_BASE = window.formipayAdmin?.restUrl || '/wp-json/formipay/v1';
+
+/**
+ * Generic AJAX request handler with nonce
+ */
+export async function ajaxRequest(action, data = {}) {
+    const formData = new FormData();
+
+    formData.append('action', action);
+    formData.append('_wpnonce', NONCE);
+
+    Object.keys(data).forEach(key => {
+        if (typeof data[key] === 'object') {
+            formData.append(key, JSON.stringify(data[key]));
+        } else {
+            formData.append(key, data[key]);
+        }
+    });
+
+    try {
+        const response = await fetch(API_BASE, {
+            method: 'POST',
+            credentials: 'same-origin',
+            body: formData,
+        });
+
+        const result = await response.json();
+
+        if (!response.ok || result.success === false) {
+            throw new Error(result.message || 'Request failed');
+        }
+
+        return result;
+    } catch (error) {
+        console.error(`AJAX error [${action}]:`, error);
+        throw error;
+    }
+}
+
+/**
+ * REST API request handler
+ */
+export async function apiRequest(endpoint, options = {}) {
+    const url = `${REST_BASE}${endpoint}`;
+
+    const defaultOptions = {
+        headers: {
+            'Content-Type': 'application/json',
+            'X-WP-Nonce': NONCE,
+        },
+    };
+
+    try {
+        const response = await fetch(url, { ...defaultOptions, ...options });
+        const result = await response.json();
+
+        if (!response.ok) {
+            throw new Error(result.message || 'API request failed');
+        }
+
+        return result;
+    } catch (error) {
+        console.error(`API error [${endpoint}]:`, error);
+        throw error;
+    }
+}
+
+/**
+ * Orders API
+ */
+export const ordersApi = {
+    list: (params = {}) => ajaxRequest('formipay_tabledata_orders', params),
+    get: (orderId) => ajaxRequest('formipay_get_order', { order_id: orderId }),
+    updateStatus: (orderId, status) => ajaxRequest('formipay_update_order_status', {
+        order_id: orderId,
+        status,
+    }),
+    delete: (orderIds) => ajaxRequest('formipay_bulk_delete_order', { ids: orderIds }),
+};
+
+/**
+ * Customers API
+ */
+export const customersApi = {
+    list: (params = {}) => ajaxRequest('formipay_tabledata_customers', params),
+    get: (customerId) => ajaxRequest('formipay_get_customer', { customer_id: customerId }),
+};
+
+/**
+ * Products API
+ */
+export const productsApi = {
+    list: (params = {}) => ajaxRequest('formipay_tabledata_products', params),
+    get: (productId) => ajaxRequest('formipay_get_product', { post_id: productId }),
+    getVariables: (productId) => ajaxRequest('get_product_variables', { post_id: productId }),
+};
+
+/**
+ * Forms API
+ */
+export const formsApi = {
+    list: (params = {}) => ajaxRequest('formipay_tabledata_forms', params),
+    get: (formId) => ajaxRequest('formipay_get_form', { post_id: formId }),
+};
+
+/**
+ * Coupons API
+ */
+export const couponsApi = {
+    list: (params = {}) => ajaxRequest('formipay_tabledata_coupons', params),
+    check: (code, formId) => ajaxRequest('formipay_check_coupon', { code, form_id: formId }),
+};
+
+/**
+ * License API
+ */
+export const licenseApi = {
+    verify: (key) => ajaxRequest('formipay_verify_license', { license_key: key }),
+    activate: (key) => ajaxRequest('formipay_activate_license', { license_key: key }),
+    deactivate: (key) => ajaxRequest('formipay_deactivate_license', { license_key: key }),
+};