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/utils/url.ts

@@ -0,0 +1,46 @@
+/**
+ * Utility functions for URL handling in production environments
+ */
+
+/**
+ * Get the base API URL, preferring relative URLs for production
+ * @returns The API base URL
+ */
+export const getApiBaseUrl = (): string => {
+  // If VITE_API_URL is explicitly set, use it
+  if (import.meta.env.VITE_API_URL && import.meta.env.VITE_API_URL !== '/api') {
+    return import.meta.env.VITE_API_URL;
+  }
+  
+  // In production, use relative URL
+  return '/api';
+};
+
+/**
+ * Build a full URL for resources (images, files, etc.)
+ * In production, this will use the current origin
+ * @param path - The resource path
+ * @returns The full URL
+ */
+export const buildResourceUrl = (path: string): string => {
+  // Remove leading slash if present
+  const cleanPath = path.startsWith('/') ? path.slice(1) : path;
+  
+  // If we have an explicit API URL that's not relative, use it
+  const apiUrl = import.meta.env.VITE_API_URL;
+  if (apiUrl && apiUrl !== '/api' && apiUrl.startsWith('http')) {
+    const baseUrl = apiUrl.replace(/\/api\/?$/, ''); // Remove /api suffix if present
+    return `${baseUrl}/${cleanPath}`;
+  }
+  
+  // In production (relative API), use current origin
+  return `${window.location.origin}/${cleanPath}`;
+};
+
+/**
+ * Check if we're in production mode (using relative URLs)
+ * @returns True if in production mode
+ */
+export const isProductionMode = (): boolean => {
+  return !import.meta.env.VITE_API_URL || import.meta.env.VITE_API_URL === '/api';
+};