feat: shopping list requires login

- Unauthenticated users see login prompt instead of shared list
- Prevents Tante Ursula from seeing Luna's Torten-Zutaten
- Shopping query only runs when authenticated
- Friendly CTA: 'Melde dich an um deine eigene Einkaufsliste zu nutzen'

frontend/src/pages/ShoppingPage.tsx

@@ -73,7 +73,7 @@ function mergeItems(groups: { recipe_title: string; recipe_id?: string; items: S
 
 export function ShoppingPage() {
   const qc = useQueryClient()
-  const { isAuthenticated } = useAuth()
+  const { isAuthenticated, isLoading: authLoading } = useAuth()
   const [newItem, setNewItem] = useState('')
   const [scope, setScope] = useState<'personal' | 'household'>('personal')
   const [viewMode, setViewMode] = useState<'recipe' | 'merged'>('merged')
@@ -100,6 +100,7 @@ export function ShoppingPage() {
     const { data: groups = [], isLoading, refetch } = useQuery({
     queryKey: ['shopping', activeScope],
     queryFn: () => fetchShopping(activeScope),
+    enabled: isAuthenticated,
   })
 
   const mergedItems = useMemo(() => mergeItems(groups), [groups])
@@ -193,7 +194,29 @@ export function ShoppingPage() {
     }
   }
 
-  if (isLoading) {
+  if (!authLoading && !isAuthenticated) {
+    return (
+      <div className="min-h-screen flex flex-col items-center justify-center px-4">
+        <div className="text-center">
+          <div className="text-6xl mb-4">🛒</div>
+          <h2 className="font-display text-2xl text-espresso mb-2">Einkaufsliste</h2>
+          <p className="text-warm-grey mb-6">
+            Melde dich an, um deine eigene Einkaufsliste zu nutzen und sie mit deinem Haushalt zu teilen.
+          </p>
+          <div className="space-y-3 w-full max-w-sm mx-auto">
+            <a href="/login" className="block w-full bg-primary text-white rounded-xl py-3 font-medium min-h-[44px] text-center">
+              Anmelden
+            </a>
+            <a href="/register" className="block w-full border border-sand text-espresso rounded-xl py-3 font-medium min-h-[44px] text-center">
+              Registrieren
+            </a>
+          </div>
+        </div>
+      </div>
+    )
+  }
+
+  if (isLoading || authLoading) {
     return (
       <div className="p-4 space-y-4">
         {[1, 2, 3].map((i) => (