Use first payment date and maturity date instead of origination date and term

Replace the term months field with a maturity date picker - users know
when their loan ends, not the month count. Compute termMonths from the
two dates. Rename "Origination Date" to "First Payment Date" and adjust
the amortization schedule so payment #1 falls on the entered date.

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

src/Purrse.Api/Services/BankSyncService.cs

@@ -665,20 +665,21 @@ public class BankSyncService : IBankSyncService
                 var originalBalance = liability.OriginalBalance ?? Math.Abs(linked.LastKnownBalance ?? 0);
                 var interestRate = liability.InterestRate ?? 0;
                 var termMonths = liability.TermMonths ?? (liability.LoanType == "Mortgage" ? 360 : 60);
-                var originationDate = liability.OriginationDate ?? DateTime.UtcNow;
+                // Plaid gives loan origination date; first payment is typically one month later
+                var firstPaymentDate = liability.OriginationDate?.AddMonths(1) ?? DateTime.UtcNow;
                 var escrowBalance = liability.EscrowBalance;
 
                 // Compute monthly payment via amortization formula if not provided
                 var monthlyPayment = liability.MonthlyPayment ?? ComputeMonthlyPayment(originalBalance, interestRate, termMonths);
 
-                var maturityDate = liability.MaturityDate ?? originationDate.AddMonths(termMonths);
+                var maturityDate = liability.MaturityDate ?? firstPaymentDate.AddMonths(termMonths - 1);
 
                 var request = new CreateLoanDetailRequest(
                     originalBalance,
                     interestRate,
                     termMonths,
                     monthlyPayment,
-                    DateTime.SpecifyKind(originationDate, DateTimeKind.Utc),
+                    DateTime.SpecifyKind(firstPaymentDate, DateTimeKind.Utc),
                     DateTime.SpecifyKind(maturityDate, DateTimeKind.Utc),
                     escrowBalance,
                     null

src/Purrse.Api/Services/LoanService.cs

@@ -187,11 +187,11 @@ public class LoanService : ILoanService
         var dailyRate = loan.InterestRate / 100m / 365m;
         var payment = loan.MonthlyPayment - (loan.EscrowAmount ?? 0);
         var extra = loan.ExtraPayment ?? 0;
-        var previousDate = loan.OriginationDate;
+        var previousDate = loan.OriginationDate.AddMonths(-1);
 
         for (int i = 1; i <= loan.TermMonths && balance > 0; i++)
         {
-            var paymentDate = loan.OriginationDate.AddMonths(i);
+            var paymentDate = loan.OriginationDate.AddMonths(i - 1);
             var days = (paymentDate - previousDate).Days;
             var interest = balance * dailyRate * days;
             var principal = Math.Min(payment + extra - interest, balance);