Auto-populate loan details from Plaid liabilities and pre-fill setup form

Fetch loan data (mortgage/student) from Plaid's Liabilities API during sync
and auto-create LoanDetail records. For accounts without liabilities data,
pre-populate the setup form with balance, interest rate, and smart term
defaults. Also fixes maturity date computation in the frontend form.

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

src/Purrse.Core/DTOs/LoanDtos.cs

@@ -37,6 +37,13 @@ public record AmortizationEntryResponse(
     decimal RemainingBalance
 );
 
+public record LoanDefaultsResponse(
+    decimal OriginalBalance,
+    decimal InterestRate,
+    int TermMonths,
+    bool HasSyncData
+);
+
 public record PayoffScenarioRequest(decimal? ExtraMonthlyPayment, decimal? LumpSumPayment, DateTime? LumpSumDate);
 
 public record PayoffScenarioResponse(