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/CompanySettings/Index.cshtml

@@ -199,6 +199,16 @@
                                     </select>
                                 </div>
                             </div>
+                            <div class="col-md-6">
+                                <div class="mb-3">
+                                    <label for="accountingMethod" class="form-label">Accounting Method</label>
+                                    <select class="form-select" id="accountingMethod" name="AccountingMethod">
+                                        <option value="1" selected="@(Model.AccountingMethod == PowderCoating.Core.Enums.AccountingMethod.Accrual ? "selected" : null)">Accrual (default)</option>
+                                        <option value="0" selected="@(Model.AccountingMethod == PowderCoating.Core.Enums.AccountingMethod.Cash ? "selected" : null)">Cash Basis</option>
+                                    </select>
+                                    <div class="form-text">Affects how financial reports (P&amp;L, Balance Sheet, Cash Flow) present data. Switching does not re-post historical transactions.</div>
+                                </div>
+                            </div>
                         </div>
 
                         <div class="mb-3">
@@ -2166,7 +2176,8 @@
                     City: $('#city').val(),
                     State: $('#state').val(),
                     ZipCode: $('#zipCode').val(),
-                    TimeZone: $('#timeZone').val()
+                    TimeZone: $('#timeZone').val(),
+                    AccountingMethod: parseInt($('#accountingMethod').val())
                 };
 
                 const btn = $('#btnSaveCompanyInfo');