fix(recipes): correct effort map casing to match backend values

effortMap had 'Easy'/'Medium'/'Hard' but the API returns 'easy'/'medium'/'hard',
so filtering by difficulty always returned nothing.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/(app)/recipes/+page.svelte

@@ -9,9 +9,9 @@
 	let activeFilter = $state('Alle');
 
 	const effortMap: Record<string, string> = {
-		Leicht: 'Easy',
+		Leicht: 'easy',
-		Mittel: 'Medium',
+		Mittel: 'medium',
-		Schwer: 'Hard'
+		Schwer: 'hard'
 	};
 
 	let filteredRecipes = $derived(