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/DTOs/Company/CompanySettingsDtos.cs

@@ -21,6 +21,7 @@ namespace PowderCoating.Application.DTOs.Company
         public string? State { get; set; }
         public string? ZipCode { get; set; }
         public string? TimeZone { get; set; }
+        public AccountingMethod AccountingMethod { get; set; } = AccountingMethod.Accrual;
         public bool HasLogo { get; set; }
 
         public CompanyOperatingCostsDto? OperatingCosts { get; set; }
@@ -96,6 +97,9 @@ namespace PowderCoating.Application.DTOs.Company
 
         [StringLength(50, ErrorMessage = "Time zone cannot exceed 50 characters")]
         public string? TimeZone { get; set; }
+
+        /// <summary>Cash or Accrual accounting method preference for financial reports.</summary>
+        public AccountingMethod AccountingMethod { get; set; } = AccountingMethod.Accrual;
     }
 
     /// <summary>