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/+server.ts

@@ -0,0 +1,112 @@
+import { json, type RequestHandler } from '@sveltejs/kit';
+import { UserFavorites } from '../../../../models/UserFavorites';
+import { Recipe } from '../../../../models/Recipe';
+import { dbConnect, dbDisconnect } from '../../../../utils/db';
+import { error } from '@sveltejs/kit';
+import mongoose from 'mongoose';
+
+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();
+    
+    await dbDisconnect();
+    
+    return json({
+      favorites: userFavorites?.favorites || []
+    });
+  } catch (e) {
+    await dbDisconnect();
+    throw error(500, 'Failed to fetch favorites');
+  }
+};
+
+export const POST: RequestHandler = async ({ request, locals }) => {
+  const session = await locals.auth();
+  
+  if (!session?.user?.nickname) {
+    throw error(401, 'Authentication required');
+  }
+  
+  const { recipeId } = await request.json();
+  
+  if (!recipeId) {
+    throw error(400, 'Recipe ID required');
+  }
+  
+  await dbConnect();
+  
+  try {
+    // Validate that the recipe exists and get its ObjectId
+    const recipe = await Recipe.findOne({ short_name: recipeId });
+    if (!recipe) {
+      await dbDisconnect();
+      throw error(404, 'Recipe not found');
+    }
+    
+    await UserFavorites.findOneAndUpdate(
+      { username: session.user.nickname },
+      { $addToSet: { favorites: recipe._id } },
+      { upsert: true, new: true }
+    );
+    
+    await dbDisconnect();
+    
+    return json({ success: true });
+  } catch (e) {
+    await dbDisconnect();
+    if (e instanceof Error && e.message.includes('404')) {
+      throw e;
+    }
+    throw error(500, 'Failed to add favorite');
+  }
+};
+
+export const DELETE: RequestHandler = async ({ request, locals }) => {
+  const session = await locals.auth();
+  
+  if (!session?.user?.nickname) {
+    throw error(401, 'Authentication required');
+  }
+  
+  const { recipeId } = await request.json();
+  
+  if (!recipeId) {
+    throw error(400, 'Recipe ID required');
+  }
+  
+  await dbConnect();
+  
+  try {
+    // Find the recipe's ObjectId
+    const recipe = await Recipe.findOne({ short_name: recipeId });
+    if (!recipe) {
+      await dbDisconnect();
+      throw error(404, 'Recipe not found');
+    }
+    
+    await UserFavorites.findOneAndUpdate(
+      { username: session.user.nickname },
+      { $pull: { favorites: recipe._id } }
+    );
+    
+    await dbDisconnect();
+    
+    return json({ success: true });
+  } catch (e) {
+    await dbDisconnect();
+    if (e instanceof Error && e.message.includes('404')) {
+      throw e;
+    }
+    throw error(500, 'Failed to remove favorite');
+  }
+};