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/PaymentSplit.ts

@@ -0,0 +1,51 @@
+import mongoose from 'mongoose';
+
+export interface IPaymentSplit {
+  _id?: string;
+  paymentId: mongoose.Schema.Types.ObjectId;
+  username: string; // username/nickname of the person who owes/is owed
+  amount: number; // amount this person owes (positive) or is owed (negative)
+  proportion?: number; // for proportional splits, the proportion (e.g., 0.5 for 50%)
+  settled: boolean; // whether this split has been settled
+  settledAt?: Date;
+  createdAt?: Date;
+  updatedAt?: Date;
+}
+
+const PaymentSplitSchema = new mongoose.Schema(
+  {
+    paymentId: { 
+      type: mongoose.Schema.Types.ObjectId, 
+      ref: 'Payment',
+      required: true 
+    },
+    username: { 
+      type: String, 
+      required: true,
+      trim: true 
+    },
+    amount: { 
+      type: Number, 
+      required: true 
+    },
+    proportion: { 
+      type: Number,
+      min: 0,
+      max: 1 
+    },
+    settled: { 
+      type: Boolean, 
+      default: false 
+    },
+    settledAt: { 
+      type: Date 
+    }
+  },
+  { 
+    timestamps: true 
+  }
+);
+
+PaymentSplitSchema.index({ paymentId: 1, username: 1 }, { unique: true });
+
+export const PaymentSplit = mongoose.model<IPaymentSplit>("PaymentSplit", PaymentSplitSchema);