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/favorites/+page.svelte

@@ -0,0 +1,58 @@
+<script lang="ts">
+    import type { PageData } from './$types';
+    import '$lib/css/nordtheme.css';
+    import Recipes from '$lib/components/Recipes.svelte';
+    import Card from '$lib/components/Card.svelte';
+    import Search from '$lib/components/Search.svelte';
+    export let data: PageData;
+    export let current_month = new Date().getMonth() + 1;
+</script>
+
+<style>
+h1{
+    text-align: center;
+    margin-bottom: 0;
+    font-size: 4rem;
+}
+.subheading{
+    text-align: center;
+    margin-top: 0;
+    font-size: 1.5rem;
+}
+.empty-state{
+    text-align: center;
+    margin-top: 3rem;
+    color: var(--nord3);
+}
+</style>
+
+<svelte:head>
+    <title>Meine Favoriten - Bocken Rezepte</title>
+    <meta name="description" content="Meine favorisierten Rezepte aus der Bockenschen Küche." />
+</svelte:head>
+
+<h1>Favoriten</h1>
+<p class=subheading>
+    {#if data.favorites.length > 0}
+        {data.favorites.length} favorisierte Rezepte
+    {:else}
+        Noch keine Favoriten gespeichert
+    {/if}
+</p>
+
+<Search></Search>
+
+{#if data.error}
+    <p class="empty-state">Fehler beim Laden der Favoriten: {data.error}</p>
+{:else if data.favorites.length > 0}
+    <Recipes>
+        {#each data.favorites as recipe}
+            <Card {recipe} {current_month}></Card>
+        {/each}
+    </Recipes>
+{:else}
+    <div class="empty-state">
+        <p>Du hast noch keine Rezepte als Favoriten gespeichert.</p>
+        <p>Besuche ein <a href="/rezepte">Rezept</a> und klicke auf das Herz-Symbol, um es zu deinen Favoriten hinzuzufügen.</p>
+    </div>
+{/if}