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>

docs/Sales Tax Report.txt

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+ I looked through the reporting code, and I do not see a dedicated sales tax report.
+
+  What exists today:
+
+  - The report menu has Financial Summary, AR Aging, Balance Sheet, and Sales & Income, but no sales-tax-specific report card in /Y:/PCC/
+    PowderCoatingApp/src/PowderCoating.Web/Views/Reports/Landing.cshtml:147 and /Y:/PCC/PowderCoatingApp/src/PowderCoating.Web/Views/Reports/
+    Landing.cshtml:211.
+  - The reporting service only exposes four finance reports in /Y:/PCC/PowderCoatingApp/src/PowderCoating.Application/Interfaces/
+    IFinancialReportService.cs:13 through /Y:/PCC/PowderCoatingApp/src/PowderCoating.Application/Interfaces/IFinancialReportService.cs:22.
+  - Sales & Income does surface tax, but only as a total and per-invoice column in /Y:/PCC/PowderCoatingApp/src/PowderCoating.Infrastructure/
+    Services/FinancialReportService.cs:406, /Y:/PCC/PowderCoatingApp/src/PowderCoating.Infrastructure/Services/FinancialReportService.cs:419,
+    and /Y:/PCC/PowderCoatingApp/src/PowderCoating.Web/Views/Reports/SalesAndIncome.cshtml:268.
+  - The good news is the underlying data is already there: invoices store TaxAmount and SalesTaxAccountId in /Y:/PCC/PowderCoatingApp/src/
+    PowderCoating.Core/Entities/Invoice.cs:23 and /Y:/PCC/PowderCoatingApp/src/PowderCoating.Core/Entities/Invoice.cs:55, and the chart of
+    accounts seeds 2200 Sales Tax Payable in /Y:/PCC/PowderCoatingApp/src/PowderCoating.Infrastructure/Services/SeedDataService.Accounts.cs:59.
+
+  Plan I’d recommend:
+
+  1. Add a new Sales Tax report under Reports > Finance with date range filters, matching the existing report pattern in /Y:/PCC/PowderCoatingApp/
+     src/PowderCoating.Web/Controllers/ReportsController.cs:993.
+  2. Build a SalesTaxReportDto plus GetSalesTaxReportAsync(...) in the reporting interface/service.
+  3. Phase 1 report contents:
+
+  - Total taxable sales
+  - Total non-taxed sales
+  - Total sales tax billed
+  - Breakdown by sales-tax liability account
+  - Breakdown by month
+  - Invoice detail grid: invoice date, invoice #, customer, subtotal, tax %, tax amount, total, amount paid, balance due, status, tax account
+
+  4. Add PDF export, and probably CSV too, since this is the kind of report people hand to accountants.
+  5. Put a report card on the Reports landing page and gate it the same way as the other accounting reports.
+  
+   Two important caveats before we build it:
+
+  - The biggest decision is whether this should be invoice-basis or cash-basis. The current model naturally supports invoice-basis first.
+  - Refunds and credit memos do not appear to store a separate tax adjustment value in /Y:/PCC/PowderCoatingApp/src/PowderCoating.Core/Entities/
+    Refund.cs:1 and /Y:/PCC/PowderCoatingApp/src/PowderCoating.Core/Entities/CreditMemo.cs:1, so phase 1 would be best framed as “sales tax billed
+    on invoices,” with net tax adjustments as a phase 2 enhancement.
+
+  My recommendation is:
+
+  - Build phase 1 as an invoice-basis Sales Tax Liability report
+  - Label zero-tax invoices as non-taxed sales
+  - Add cash-basis and refund/credit tax adjustments later if you need filing-grade accuracy for more complex cases
+
+  If you want, I can turn this plan into the actual report next.