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/api/rezepte/favorites/recipes/+server.ts

@@ -0,0 +1,40 @@
+import { json, type RequestHandler } from '@sveltejs/kit';
+import { UserFavorites } from '../../../../../models/UserFavorites';
+import { Recipe } from '../../../../../models/Recipe';
+import { dbConnect, dbDisconnect } from '../../../../../utils/db';
+import type { RecipeModelType } from '../../../../../types/types';
+import { error } from '@sveltejs/kit';
+
+export const GET: RequestHandler = async ({ locals }) => {
+  const session = await locals.auth();
+  
+  if (!session?.user?.nickname) {
+    throw error(401, 'Authentication required');
+  }
+  
+  await dbConnect();
+  
+  try {
+    const userFavorites = await UserFavorites.findOne({ 
+      username: session.user.nickname 
+    }).lean();
+    
+    if (!userFavorites?.favorites?.length) {
+      await dbDisconnect();
+      return json([]);
+    }
+    
+    let recipes = await Recipe.find({ 
+      _id: { $in: userFavorites.favorites }
+    }).lean() as RecipeModelType[];
+    
+    await dbDisconnect();
+    
+    recipes = JSON.parse(JSON.stringify(recipes));
+    
+    return json(recipes);
+  } catch (e) {
+    await dbDisconnect();
+    throw error(500, 'Failed to fetch favorite recipes');
+  }
+};