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/lib/assets/icons/Heart.svelte

@@ -0,0 +1,33 @@
+<script>
+</script>
+<style>
+@keyframes shake{
+    0%{
+      transform: rotate(0)
+		scale(1,1);
+    }
+    25%{
+    	box-shadow: 0em 0em 1em 0.2em rgba(0, 0, 0, 0.6);
+      	transform: rotate(30deg)
+      		scale(1.2,1.2)
+      ;
+    }
+    50%{
+
+    	box-shadow: 0em 0em 1em 0.2em rgba(0, 0, 0, 0.6);
+      	transform: rotate(-30deg)
+      		scale(1.2,1.2);
+    }
+    74%{
+
+    	box-shadow: 0em 0em 1em 0.2em rgba(0, 0, 0, 0.6);
+  	transform: rotate(30deg)
+  		scale(1.2, 1.2);
+	}
+	100%{
+      transform: rotate(0)
+      	scale(1,1);
+    }
+  }
+</style>
+<svg xmlns="http://www.w3.org/2000/svg" height="1em" viewBox="0 0 512 512" {...$$restProps}><!--! Font Awesome Free 6.4.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license (Commercial License) Copyright 2023 Fonticons, Inc. --><path d="m47.6 300.4 180.7 168.7c7.5 7 17.4 10.9 27.7 10.9s20.2-3.9 27.7-10.9L464.4 300.4c30.4-28.3 47.6-68 47.6-109.5v-5.8c0-69.9-50.5-129.5-119.4-141C347 36.5 300.6 51.4 268 84L256 96 244 84c-32.6-32.6-79-47.5-124.6-39.9C50.5 55.6 0 115.2 0 185.1v5.8c0 41.5 17.2 81.2 47.6 109.5z"/></svg>