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/components/FavoriteButton.svelte

@@ -0,0 +1,102 @@
+<script lang="ts">
+  import Heart from '$lib/assets/icons/Heart.svelte';
+  
+  export let recipeId: string;
+  export let isFavorite: boolean = false;
+  export let isLoggedIn: boolean = false;
+  
+  let isLoading = false;
+
+  async function toggleFavorite() {
+    if (!isLoggedIn || isLoading) return;
+    
+    isLoading = true;
+    
+    try {
+      const method = isFavorite ? 'DELETE' : 'POST';
+      const response = await fetch('/api/rezepte/favorites', {
+        method,
+        headers: {
+          'Content-Type': 'application/json',
+        },
+        body: JSON.stringify({ recipeId }),
+      });
+      
+      if (response.ok) {
+        isFavorite = !isFavorite;
+      }
+    } catch (error) {
+      console.error('Failed to toggle favorite:', error);
+    } finally {
+      isLoading = false;
+    }
+  }
+</script>
+
+<style>
+  .favorite-button {
+    all: unset;
+    color: var(--nord0);
+    font-size: 1.1rem;
+    background-color: var(--nord5);
+    border-radius: 10000px;
+    padding: 0.5em 1em;
+    transition: 100ms;
+    box-shadow: 0em 0em 0.5em 0.05em rgba(0,0,0,0.3);
+    display: flex;
+    align-items: center;
+    gap: 0.5em;
+    cursor: pointer;
+  }
+  
+  .favorite-button:disabled {
+    opacity: 0.6;
+    cursor: not-allowed;
+  }
+  
+  .favorite-button.favorited {
+    background-color: var(--nord11);
+    color: white;
+  }
+  
+  .favorite-button:not(.favorited):hover,
+  .favorite-button:not(.favorited):focus-visible {
+    transform: scale(1.1);
+    background-color: var(--nord11);
+    color: white;
+    box-shadow: 0.1em 0.1em 0.5em 0.1em rgba(0,0,0,0.3);
+  }
+  
+  .favorite-button.favorited:hover,
+  .favorite-button.favorited:focus-visible {
+    transform: scale(1.1);
+    background-color: var(--nord5);
+    color: var(--nord0);
+    box-shadow: 0.1em 0.1em 0.5em 0.1em rgba(0,0,0,0.3);
+  }
+  
+  @media (prefers-color-scheme: dark) {
+    .favorite-button {
+      background-color: var(--nord0);
+      color: white;
+    }
+    
+    .favorite-button.favorited:hover,
+    .favorite-button.favorited:focus-visible {
+      background-color: var(--nord6);
+      color: var(--nord0);
+    }
+  }
+</style>
+
+{#if isLoggedIn}
+  <button 
+    class="favorite-button" 
+    class:favorited={isFavorite}
+    disabled={isLoading}
+    on:click={toggleFavorite}
+  >
+    <Heart />
+    {isFavorite ? 'Favorit entfernen' : 'Als Favorit speichern'}
+  </button>
+{/if}