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/components/admin/ThemeCustomizer.tsx

@@ -4,6 +4,7 @@ import { Button, Card, Input } from '../common';
 import { GALLERY_THEME_PRESETS, type ThemeConfig } from '../../contexts/ThemeContext';
 import { settingsService } from '../../services/settings.service';
 import { toast } from 'react-toastify';
+import { buildResourceUrl } from '../../utils/url';
 
 interface ThemeCustomizerProps {
   value: ThemeConfig;
@@ -269,7 +270,7 @@ export const ThemeCustomizer: React.FC<ThemeCustomizerProps> = ({
             <div className="flex items-center gap-4">
               {localTheme.logoUrl && (
                 <img
-                  src={localTheme.logoUrl.startsWith('http') ? localTheme.logoUrl : `${import.meta.env.VITE_API_URL || 'http://localhost:3001'}${localTheme.logoUrl}`}
+                  src={localTheme.logoUrl.startsWith('http') ? localTheme.logoUrl : buildResourceUrl(localTheme.logoUrl)}
                   alt="Custom logo"
                   className="h-16 w-auto object-contain"
                 />