Add Navigation component and integrate it into multiple pages for consistent navigation

frontend/app/admin/products/page.tsx

@@ -2,12 +2,14 @@ import { fetchJson } from '../../../lib/api';
 import type { Product } from '../../../features/inventory/types';
 import MergePreviewForm from './MergePreviewForm';
 import AdminProductList from './AdminProductList';
+import Navigation from '../../Navigation';
 
 export default async function AdminProductsPage() {
   const products = await fetchJson<Product[]>('/api/products');
 
   return (
-    <main style={{ padding: '1.5rem', maxWidth: '1100px', margin: '0 auto' }}>
+    <main style={{ padding: '1rem', maxWidth: '1100px', margin: '0 auto' }}>
+      <Navigation />
       <h1 style={{ marginBottom: '1.5rem' }}>Admin: Produkter</h1>
       <p>Här kan du granska och standardisera produktnamn.</p>