Implement user favorites feature for recipes

- Add UserFavorites MongoDB model with ObjectId references
- Create authenticated API endpoints for favorites management
- Add Heart icon and FavoriteButton components with toggle functionality
- Display favorite button below recipe tags for logged-in users
- Add Favoriten navigation link (visible only when authenticated)
- Create favorites page with grid layout and search functionality
- Store favorites by MongoDB ObjectId for data integrity

🤖 Generated with [Claude Code](https://claude.ai/code)

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

src/routes/rezepte/+layout.svelte

@@ -11,6 +11,9 @@ if(data.session){
 <Header>
 	<ul class=site_header slot=links>
 	<li><a href="/rezepte">Alle Rezepte</a></li>
+	{#if user}
+		<li><a href="/rezepte/favorites">Favoriten</a></li>
+	{/if}
 	<li><a href="/rezepte/season">In Saison</a></li>
 	<li><a href="/rezepte/category">Kategorie</a></li>
 	<li><a href="/rezepte/icon">Icon</a></li>