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/upload/+server.ts

@@ -0,0 +1,63 @@
+import type { RequestHandler } from '@sveltejs/kit';
+import { error, json } from '@sveltejs/kit';
+import { writeFileSync, mkdirSync } from 'fs';
+import { join } from 'path';
+import { randomUUID } from 'crypto';
+import { IMAGE_DIR } from '$env/static/private';
+
+export const POST: RequestHandler = async ({ request, locals }) => {
+  const auth = await locals.auth();
+  if (!auth || !auth.user?.nickname) {
+    throw error(401, 'Not logged in');
+  }
+
+  try {
+    const formData = await request.formData();
+    const image = formData.get('image') as File;
+    
+    if (!image) {
+      throw error(400, 'No image provided');
+    }
+
+    const allowedTypes = ['image/jpeg', 'image/jpg', 'image/png', 'image/webp'];
+    if (!allowedTypes.includes(image.type)) {
+      throw error(400, 'Invalid file type. Only JPEG, PNG, and WebP are allowed.');
+    }
+
+    if (image.size > 5 * 1024 * 1024) {
+      throw error(400, 'File too large. Maximum size is 5MB.');
+    }
+
+    const extension = image.type.split('/')[1];
+    const filename = `${randomUUID()}.${extension}`;
+    
+    if (!IMAGE_DIR) {
+      throw error(500, 'IMAGE_DIR environment variable not configured');
+    }
+    
+    // Ensure cospend directory exists in IMAGE_DIR
+    const uploadsDir = join(IMAGE_DIR, 'cospend');
+    try {
+      mkdirSync(uploadsDir, { recursive: true });
+    } catch (err) {
+      // Directory might already exist
+    }
+
+    const filepath = join(uploadsDir, filename);
+    const buffer = await image.arrayBuffer();
+    
+    writeFileSync(filepath, new Uint8Array(buffer));
+    
+    const publicPath = `/cospend/${filename}`;
+    
+    return json({
+      success: true,
+      path: publicPath
+    });
+
+  } catch (err) {
+    if (err.status) throw err;
+    console.error('Upload error:', err);
+    throw error(500, 'Failed to upload file');
+  }
+};