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.Infrastructure/Migrations/ApplicationDbContextModelSnapshot.cs

@@ -1546,6 +1546,9 @@ namespace PowderCoating.Infrastructure.Migrations
 
                     SqlServerPropertyBuilderExtensions.UseIdentityColumn(b.Property<int>("Id"));
 
+                    b.Property<int>("AccountingMethod")
+                        .HasColumnType("int");
+
                     b.Property<bool?>("AccountingOverride")
                         .HasColumnType("bit");
 
@@ -6077,7 +6080,7 @@ namespace PowderCoating.Infrastructure.Migrations
                         {
                             Id = 1,
                             CompanyId = 0,
-                            CreatedAt = new DateTime(2026, 5, 10, 1, 12, 48, 386, DateTimeKind.Utc).AddTicks(2249),
+                            CreatedAt = new DateTime(2026, 5, 10, 3, 25, 9, 644, DateTimeKind.Utc).AddTicks(9957),
                             Description = "Standard pricing for regular customers",
                             DiscountPercent = 0m,
                             IsActive = true,
@@ -6088,7 +6091,7 @@ namespace PowderCoating.Infrastructure.Migrations
                         {
                             Id = 2,
                             CompanyId = 0,
-                            CreatedAt = new DateTime(2026, 5, 10, 1, 12, 48, 386, DateTimeKind.Utc).AddTicks(2260),
+                            CreatedAt = new DateTime(2026, 5, 10, 3, 25, 9, 644, DateTimeKind.Utc).AddTicks(9963),
                             Description = "5% discount for preferred customers",
                             DiscountPercent = 5m,
                             IsActive = true,
@@ -6099,7 +6102,7 @@ namespace PowderCoating.Infrastructure.Migrations
                         {
                             Id = 3,
                             CompanyId = 0,
-                            CreatedAt = new DateTime(2026, 5, 10, 1, 12, 48, 386, DateTimeKind.Utc).AddTicks(2261),
+                            CreatedAt = new DateTime(2026, 5, 10, 3, 25, 9, 644, DateTimeKind.Utc).AddTicks(9965),
                             Description = "10% discount for premium customers",
                             DiscountPercent = 10m,
                             IsActive = true,