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/Interfaces/Services/ILoanService.cs

@@ -9,4 +9,5 @@ public interface ILoanService
     Task<LoanDetailResponse> UpdateLoanDetailAsync(Guid userId, Guid accountId, CreateLoanDetailRequest request);
     Task<List<AmortizationEntryResponse>> GetAmortizationScheduleAsync(Guid userId, Guid accountId);
     Task<PayoffScenarioResponse> CalculatePayoffScenarioAsync(Guid userId, Guid accountId, PayoffScenarioRequest request);
+    Task<LoanDefaultsResponse> GetLoanDefaultsAsync(Guid userId, Guid accountId);
 }