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.Application/Interfaces/IPdfService.cs

@@ -41,6 +41,7 @@ public interface IPdfService
     Task<byte[]> GenerateBalanceSheetPdfAsync(BalanceSheetDto dto);
     Task<byte[]> GenerateArAgingPdfAsync(ArAgingReportDto dto);
     Task<byte[]> GenerateSalesAndIncomePdfAsync(SalesIncomeReportDto dto);
+    Task<byte[]> GenerateSalesTaxReportPdfAsync(SalesTaxReportDto dto);
 
     Task<byte[]> GenerateGiftCertificatePdfAsync(
         GiftCertificateDto cert,