Don't pre-populate original balance from current balance in loan defaults

The current account balance is not the origination balance, so defaulting
it into the Original Balance field is misleading. Leave it at 0 so users
enter the actual loan origination amount.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/Purrse.Api/Services/LoanService.cs

@@ -162,7 +162,6 @@ public class LoanService : ILoanService
             .FirstOrDefaultAsync(a => a.Id == accountId && a.UserId == userId)
             ?? throw new KeyNotFoundException("Account not found");
 
-        var originalBalance = Math.Abs(account.Balance);
         var interestRate = account.InterestRate ?? 0;
         var hasSyncData = account.LinkedAccounts.Any() && (account.InterestRate.HasValue || account.Balance != 0);
 
@@ -174,7 +173,7 @@ public class LoanService : ILoanService
             _ => 360
         };
 
-        return new LoanDefaultsResponse(originalBalance, interestRate, termMonths, hasSyncData);
+        return new LoanDefaultsResponse(0, interestRate, termMonths, hasSyncData);
     }
 
     private static List<AmortizationEntry> GenerateAmortizationSchedule(LoanDetail loan)