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/models/Payment.ts

@@ -0,0 +1,79 @@
+import mongoose from 'mongoose';
+
+export interface IPayment {
+  _id?: string;
+  title: string;
+  description?: string;
+  amount: number;
+  currency: string;
+  paidBy: string; // username/nickname of the person who paid
+  date: Date;
+  image?: string; // path to uploaded image
+  splitMethod: 'equal' | 'full' | 'proportional';
+  createdBy: string; // username/nickname of the person who created the payment
+  createdAt?: Date;
+  updatedAt?: Date;
+}
+
+const PaymentSchema = new mongoose.Schema(
+  {
+    title: { 
+      type: String, 
+      required: true,
+      trim: true 
+    },
+    description: { 
+      type: String,
+      trim: true 
+    },
+    amount: { 
+      type: Number, 
+      required: true,
+      min: 0 
+    },
+    currency: { 
+      type: String, 
+      required: true, 
+      default: 'CHF',
+      enum: ['CHF'] // For now only CHF as requested
+    },
+    paidBy: { 
+      type: String, 
+      required: true,
+      trim: true 
+    },
+    date: { 
+      type: Date, 
+      required: true,
+      default: Date.now 
+    },
+    image: { 
+      type: String,
+      trim: true 
+    },
+    splitMethod: {
+      type: String,
+      required: true,
+      enum: ['equal', 'full', 'proportional'],
+      default: 'equal'
+    },
+    createdBy: { 
+      type: String, 
+      required: true,
+      trim: true 
+    }
+  },
+  { 
+    timestamps: true,
+    toJSON: { virtuals: true },
+    toObject: { virtuals: true }
+  }
+);
+
+PaymentSchema.virtual('splits', {
+  ref: 'PaymentSplit',
+  localField: '_id',
+  foreignField: 'paymentId'
+});
+
+export const Payment = mongoose.model<IPayment>("Payment", PaymentSchema);