Add Phase A accounting features: AP Aging, Trial Balance, Cash vs Accrual

- AP Aging report (GetApAgingAsync, controller actions, view, PDF export)
  mirrors AR Aging — groups open bills by vendor, buckets by days past due date
- Trial Balance report (GetTrialBalanceAsync, view, PDF export)
  uses Account.CurrentBalance, groups by AccountType, validates debits == credits
- Cash vs Accrual accounting method setting on Company entity
  switchable at any time — report-time only, no GL re-posting on change
  P&L cash: revenue = payments received; expenses = bills/expenses paid in period
  Balance Sheet cash: omits AR and AP lines (no receivables/payables concept)
  AccountingMethod badge shown on P&L and Balance Sheet views
- Migration A (AddAccountingMethod) applied, default = Accrual for all existing companies
- AP Aging and Trial Balance added to Reports Landing page

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

src/PowderCoating.Application/Interfaces/IPdfService.cs

@@ -42,6 +42,8 @@ public interface IPdfService
     Task<byte[]> GenerateArAgingPdfAsync(ArAgingReportDto dto);
     Task<byte[]> GenerateSalesAndIncomePdfAsync(SalesIncomeReportDto dto);
     Task<byte[]> GenerateSalesTaxReportPdfAsync(SalesTaxReportDto dto);
+    Task<byte[]> GenerateApAgingPdfAsync(ApAgingReportDto dto);
+    Task<byte[]> GenerateTrialBalancePdfAsync(TrialBalanceDto dto);
 
     Task<byte[]> GenerateGiftCertificatePdfAsync(
         GiftCertificateDto cert,