Enhance Cospend with debt breakdown and predefined users

- Add EnhancedBalance component with integrated single-user debt display
- Create DebtBreakdown component for multi-user debt overview
- Add predefined users configuration (alexander, anna)
- Implement personal + equal split payment method
- Add profile pictures throughout payment interfaces
- Integrate debt information with profile pictures in balance view
- Auto-hide debt breakdown when single user (shows in balance instead)
- Support both manual and predefined user management modes

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

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

src/models/PaymentSplit.ts

@@ -6,6 +6,7 @@ export interface IPaymentSplit {
   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%)
+  personalAmount?: number; // for personal_equal splits, the personal portion for this user
   settled: boolean; // whether this split has been settled
   settledAt?: Date;
   createdAt?: Date;
@@ -33,6 +34,10 @@ const PaymentSplitSchema = new mongoose.Schema(
       min: 0,
       max: 1 
     },
+    personalAmount: {
+      type: Number,
+      min: 0
+    },
     settled: { 
       type: Boolean, 
       default: false