Add Sales Tax Liability report with PDF and CSV export

Invoice-basis report showing taxable vs non-taxable sales, tax billed
by GL account, monthly trend table/chart, and full invoice detail grid.
Non-taxable invoice rows shaded grey for easy scanning. Quick-preset
date buttons (This Month, Last Month, YTD, Last Year) for common filing
periods. CSV export formatted for accountants and tax-filing software.
Gated behind AllowAccounting() like other financial reports.

- SalesTaxReportDto + 3 supporting DTOs in FinancialReportDtos.cs
- GetSalesTaxReportAsync on IFinancialReportService + implementation
- GenerateSalesTaxReportPdfAsync on IPdfService + QuestPDF implementation
- SalesTax / SalesTaxPdf / SalesTaxCsv actions in ReportsController
- Views/Reports/SalesTax.cshtml with Chart.js monthly trend chart
- Landing page card added to Finance section
- HelpKnowledgeBase and Help/Reports.cshtml updated with full docs

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/PowderCoating.Web/Views/Reports/Landing.cshtml

@@ -212,6 +212,14 @@
             <p>Detailed breakdown of sales by customer, job type, and period.</p>
             <div class="report-arrow">Open report <i class="bi bi-arrow-right"></i></div>
         </a>
+        <a asp-controller="Reports" asp-action="SalesTax" class="report-card">
+            <div class="report-card-icon" style="background:#faf5ff;color:#7c3aed;">
+                <i class="bi bi-percent"></i>
+            </div>
+            <h5>Sales Tax Report</h5>
+            <p>Invoice-basis tax liability: taxable vs non-taxable sales, tax billed by account and month, full invoice detail.</p>
+            <div class="report-arrow">Open report <i class="bi bi-arrow-right"></i></div>
+        </a>
         <a asp-controller="Reports" asp-action="CustomerOverview" class="report-card">
             <div class="report-card-icon" style="background:#eff6ff;color:#2563eb;">
                 <i class="bi bi-people"></i>