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

frontend/app/inventory/page.tsx

@@ -4,6 +4,7 @@ import Link from 'next/link';
 import { fetchJson } from '../../lib/api';
 import type { InventoryItem, Product } from '../../features/inventory/types';
 import InventoryList from './InventoryList';
+import Navigation from '../Navigation';
 
 function formatDate(value: string | null) {
   if (!value) return null;
@@ -104,7 +105,8 @@ export default async function InventoryPage({ searchParams }: InventoryPageProps
   ];
 
   return (
-    <main style={{ padding: '1.5rem', maxWidth: '1000px', margin: '0 auto' }}>
+    <main style={{ padding: '1rem', maxWidth: '1000px', margin: '0 auto' }}>
+      <Navigation />
       <h1 style={{ marginBottom: '1.5rem' }}>Varor hemma</h1>
 
       <ProductForm />