feat(api): enhance fetchJson to include authentication headers for server requests

frontend/lib/api.ts

@@ -1,17 +1,23 @@
+import { getAuthHeaders } from './auth-headers';
+
 const API_BASE =
   process.env.NEXT_PUBLIC_API_URL_INTERNAL || 'http://recipe-api:8080';
 
 export async function fetchJson<T>(path: string, init?: RequestInit): Promise<T> {
   // Använd alltid relativ path i webbläsaren för att undvika mixed content
-  const url = typeof window === 'undefined'
+  const isServer = typeof window === 'undefined';
+  const url = isServer
     ? (path.startsWith('http') ? path : `${API_BASE}${path}`)
     : path;
 
+  const authHeaders = isServer ? await getAuthHeaders() : {};
+
   const res = await fetch(url, {
     ...init,
     cache: 'no-store',
     headers: {
       'Content-Type': 'application/json',
+      ...authHeaders,
       ...(init?.headers || {}),
     },
   });