Add complete Cospend expense sharing feature

- Add MongoDB models for Payment and PaymentSplit with proper splitting logic
- Implement API routes for CRUD operations and balance calculations
- Create dashboard with balance overview and recent activity
- Add payment creation form with file upload (using $IMAGE_DIR)
- Implement shallow routing with modal side panel for payment details
- Support multiple split methods: equal, full payment, custom proportions
- Add responsive design for desktop and mobile
- Integrate with existing Authentik authentication

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/routes/api/cospend/payments/+server.ts

@@ -0,0 +1,92 @@
+import type { RequestHandler } from '@sveltejs/kit';
+import { Payment } from '../../../../models/Payment';
+import { PaymentSplit } from '../../../../models/PaymentSplit';
+import { dbConnect, dbDisconnect } from '../../../../utils/db';
+import { error, json } from '@sveltejs/kit';
+
+export const GET: RequestHandler = async ({ locals, url }) => {
+  const auth = await locals.auth();
+  if (!auth || !auth.user?.nickname) {
+    throw error(401, 'Not logged in');
+  }
+
+  const limit = parseInt(url.searchParams.get('limit') || '20');
+  const offset = parseInt(url.searchParams.get('offset') || '0');
+
+  await dbConnect();
+  
+  try {
+    const payments = await Payment.find()
+      .populate('splits')
+      .sort({ date: -1 })
+      .limit(limit)
+      .skip(offset)
+      .lean();
+
+    return json({ payments });
+  } catch (e) {
+    throw error(500, 'Failed to fetch payments');
+  } finally {
+    await dbDisconnect();
+  }
+};
+
+export const POST: RequestHandler = async ({ request, locals }) => {
+  const auth = await locals.auth();
+  if (!auth || !auth.user?.nickname) {
+    throw error(401, 'Not logged in');
+  }
+
+  const data = await request.json();
+  const { title, description, amount, paidBy, date, image, splitMethod, splits } = data;
+
+  if (!title || !amount || !paidBy || !splitMethod || !splits) {
+    throw error(400, 'Missing required fields');
+  }
+
+  if (amount <= 0) {
+    throw error(400, 'Amount must be positive');
+  }
+
+  if (!['equal', 'full', 'proportional'].includes(splitMethod)) {
+    throw error(400, 'Invalid split method');
+  }
+
+  await dbConnect();
+  
+  try {
+    const payment = await Payment.create({
+      title,
+      description,
+      amount,
+      currency: 'CHF',
+      paidBy,
+      date: date ? new Date(date) : new Date(),
+      image,
+      splitMethod,
+      createdBy: auth.user.nickname
+    });
+
+    const splitPromises = splits.map((split: any) => {
+      return PaymentSplit.create({
+        paymentId: payment._id,
+        username: split.username,
+        amount: split.amount,
+        proportion: split.proportion
+      });
+    });
+
+    await Promise.all(splitPromises);
+
+    return json({ 
+      success: true, 
+      payment: payment._id 
+    });
+
+  } catch (e) {
+    console.error('Error creating payment:', e);
+    throw error(500, 'Failed to create payment');
+  } finally {
+    await dbDisconnect();
+  }
+};