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/cospend/payments/add/+page.server.ts

@@ -0,0 +1,14 @@
+import type { PageServerLoad } from './$types';
+import { redirect } from '@sveltejs/kit';
+
+export const load: PageServerLoad = async ({ locals }) => {
+  const session = await locals.auth();
+  
+  if (!session) {
+    throw redirect(302, '/login');
+  }
+
+  return {
+    session
+  };
+};