feat: Enhance server security and performance by adding helmet for security headers, implementing rate limiting, and centralizing error handling; update frontend API calls to use a dedicated api module for consistency

frontend/src/BillingManager.jsx

@@ -1,5 +1,5 @@
 import { useState, useEffect, useRef } from 'react';
-import axios from 'axios';
+import api from './lib/api.js';
 import { 
   IconPlus, 
   IconEdit, 
@@ -114,7 +114,7 @@ function BillingManager() {
   const fetchBillingData = async () => {
     setLoading(true);
     try {
-      const response = await axios.get(`${API_URL}/billing`);
+      const response = await api.get(`/billing`);
       const normalized = (response.data || []).map((item, idx) => ({
         id: item.id ?? String(idx + 1),
         payments: Array.isArray(item.payments)
@@ -139,7 +139,7 @@ function BillingManager() {
   const handleSaveChanges = async () => {
     setLoading(true);
     try {
-      await axios.post(`${API_URL}/billing`, { domains: billingData });
+      await api.post(`/billing`, { domains: billingData });
       setSuccess('Данные успешно сохранены');
       setTimeout(() => setSuccess(''), 3000);
     } catch (err) {