fix: remove hardcoded localhost URLs for production deployment

- Add URL utility functions for building resource URLs
- Update all components to use relative URLs in production
- Add production deployment documentation
- Update nginx config to proxy all required endpoints
- Add .env.production.example with proper configuration

frontend/src/contexts/MaintenanceContext.tsx

@@ -1,6 +1,7 @@
 import React, { createContext, useContext, useState, useEffect, ReactNode } from 'react';
 import { useQuery } from '@tanstack/react-query';
 import { setMaintenanceModeCallback } from '../config/api';
+import { getApiBaseUrl } from '../utils/url';
 
 interface MaintenanceContextType {
   isMaintenanceMode: boolean;
@@ -29,7 +30,7 @@ export const MaintenanceProvider: React.FC<MaintenanceProviderProps> = ({ childr
     queryKey: ['public-settings-maintenance'],
     queryFn: async () => {
       try {
-        const response = await fetch(`${import.meta.env.VITE_API_URL || 'http://localhost:3001'}/api/public/settings`);
+        const response = await fetch(`${getApiBaseUrl()}/public/settings`);
         if (response.status === 503) {
           setIsMaintenanceMode(true);
           return null;