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/DataManager.jsx

@@ -1,5 +1,5 @@
 import { useState, useEffect, useRef } from 'react';
-import axios from 'axios';
+import api from './lib/api.js';
 import { 
   IconPlus, 
   IconSearch, 
@@ -44,7 +44,7 @@ function DataManager({ entityName, entityKey, placeholder }) {
   const fetchItems = async () => {
     setLoading(true);
     try {
-      const response = await axios.get(`${API_URL}/${entityKey}`);
+      const response = await api.get(`/${entityKey}`);
       setItems(response.data);
       setError('');
     } catch (error) {
@@ -102,7 +102,7 @@ function DataManager({ entityName, entityKey, placeholder }) {
   const handleSaveChanges = async () => {
     setLoading(true);
     try {
-      await axios.post(`${API_URL}/${entityKey}`, { domains: items });
+      await api.post(`/${entityKey}`, { domains: items });
       setSuccess('Изменения успешно сохранены!');
       setTimeout(() => setSuccess(''), 3000);
     } catch (error) {