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.Web/Views/Reports/ProfitAndLoss.cshtml

@@ -30,6 +30,14 @@
 <div class="d-flex align-items-center gap-2 mb-3 no-print">
     <a asp-action="Index" class="btn btn-sm btn-outline-secondary"><i class="bi bi-arrow-left"></i></a>
     <p class="text-muted mb-0">Income Statement — @Model.From.ToString("MMM d") – @Model.To.ToString("MMM d, yyyy")</p>
+    @if (Model.AccountingMethod == PowderCoating.Core.Enums.AccountingMethod.Cash)
+    {
+        <span class="badge bg-warning text-dark">Cash Basis</span>
+    }
+    else
+    {
+        <span class="badge bg-info text-dark">Accrual Basis</span>
+    }
     <div class="ms-auto d-flex gap-2">
         <a href="@Url.Action("ProfitAndLossPdf", new { from = Model.From.ToString("yyyy-MM-dd"), to = Model.To.ToString("yyyy-MM-dd") })"
            class="btn btn-sm btn-outline-danger no-print" target="_blank">