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/DTOs/Accounting/FinancialReportDtos.cs

@@ -159,3 +159,65 @@ public class SalesInvoiceLineDto
     public decimal AmountPaid { get; set; }
     public decimal BalanceDue { get; set; }
 }
+
+// ============================================================
+// SALES TAX REPORT
+// ============================================================
+
+public class SalesTaxReportDto
+{
+    public DateTime From { get; set; }
+    public DateTime To { get; set; }
+    public string CompanyName { get; set; } = string.Empty;
+
+    /// <summary>Subtotal of invoices where TaxAmount > 0.</summary>
+    public decimal TotalTaxableSales { get; set; }
+    /// <summary>Subtotal of invoices where TaxAmount == 0.</summary>
+    public decimal TotalNonTaxableSales { get; set; }
+    /// <summary>Sum of all TaxAmount values across the period.</summary>
+    public decimal TotalTaxBilled { get; set; }
+    public int TaxableInvoiceCount { get; set; }
+    public int NonTaxableInvoiceCount { get; set; }
+    public decimal EffectiveTaxRate => TotalTaxableSales == 0 ? 0
+        : Math.Round(TotalTaxBilled / TotalTaxableSales * 100, 2);
+
+    public List<SalesTaxByAccountDto> ByAccount { get; set; } = new();
+    public List<SalesTaxByMonthDto> ByMonth { get; set; } = new();
+    public List<SalesTaxInvoiceLineDto> Invoices { get; set; } = new();
+}
+
+public class SalesTaxByAccountDto
+{
+    public int? AccountId { get; set; }
+    public string AccountName { get; set; } = string.Empty;
+    public string AccountNumber { get; set; } = string.Empty;
+    public decimal TaxableSales { get; set; }
+    public decimal TaxBilled { get; set; }
+    public int InvoiceCount { get; set; }
+}
+
+public class SalesTaxByMonthDto
+{
+    public int Year { get; set; }
+    public int Month { get; set; }
+    public string Label { get; set; } = string.Empty;
+    public decimal TaxableSales { get; set; }
+    public decimal TaxBilled { get; set; }
+    public int InvoiceCount { get; set; }
+}
+
+public class SalesTaxInvoiceLineDto
+{
+    public int InvoiceId { get; set; }
+    public string InvoiceNumber { get; set; } = string.Empty;
+    public string CustomerName { get; set; } = string.Empty;
+    public DateTime InvoiceDate { get; set; }
+    public string Status { get; set; } = string.Empty;
+    public decimal SubTotal { get; set; }
+    public decimal TaxPercent { get; set; }
+    public decimal TaxAmount { get; set; }
+    public decimal Total { get; set; }
+    public decimal AmountPaid { get; set; }
+    public decimal BalanceDue { get; set; }
+    public string TaxAccountName { get; set; } = string.Empty;
+}