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/Landing.cshtml

@@ -188,6 +188,22 @@
             <p>Snapshot of assets, liabilities, and equity as of any date.</p>
             <div class="report-arrow">Open report <i class="bi bi-arrow-right"></i></div>
         </a>
+        <a asp-controller="Reports" asp-action="ApAging" class="report-card">
+            <div class="report-card-icon" style="background:#fff1f2;color:#b91c1c;">
+                <i class="bi bi-hourglass-split"></i>
+            </div>
+            <h5>AP Aging</h5>
+            <p>Outstanding vendor bills by age — current, 30, 60, and 90+ days past due. Exportable to PDF.</p>
+            <div class="report-arrow">Open report <i class="bi bi-arrow-right"></i></div>
+        </a>
+        <a asp-controller="Reports" asp-action="TrialBalance" class="report-card">
+            <div class="report-card-icon" style="background:#eef2ff;color:#4338ca;">
+                <i class="bi bi-list-columns-reverse"></i>
+            </div>
+            <h5>Trial Balance</h5>
+            <p>All active accounts with debit and credit balances — validates that your books are in balance.</p>
+            <div class="report-arrow">Open report <i class="bi bi-arrow-right"></i></div>
+        </a>
     </div>
 </div>
 }