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/Helpers/HelpKnowledgeBase.cs

@@ -569,6 +569,7 @@ public static class HelpKnowledgeBase
         - *Balance Sheet* — assets, liabilities, equity snapshot
         - *AR Aging* — outstanding invoices grouped by age (0-30, 31-60, 61-90, 90+ days)
         - *Sales & Income* — revenue trends by period
+        - *Sales Tax Report* — invoice-basis tax liability: taxable vs non-taxable sales, tax billed by account and by month, full invoice detail grid. Supports PDF export and CSV export (for handing to your accountant or tax software). Found under Reports → Finance.
         - *Revenue Trends* — monthly/quarterly revenue charting
         - *Operations Report* — job throughput, cycle times, status breakdown
         - *Customer Overview* — top customers, revenue per customer
@@ -579,7 +580,7 @@ public static class HelpKnowledgeBase
         - *Powder Usage Report* — powder consumption by item/job
         - *Job Cycle Time Report* — how long jobs spend in each status
 
-        Most financial reports support PDF export.
+        Most financial reports support PDF export. The Sales Tax Report also supports CSV export.
 
         ---
 
@@ -1216,7 +1217,7 @@ public static class HelpKnowledgeBase
 
         The system includes several AI-powered features (all use Claude by Anthropic):
 
-        1. **AI Photo Quote** — Upload photos of items on a quote; AI estimates surface area, complexity, and labor time. Available in the quote item wizard. *Availability depends on your subscription plan.* The AI is tuned for consistency — running the same photo through the wizard multiple times should produce very similar estimates each time (small differences may occur due to the visual nature of photo analysis, but the numbers will be in the same ballpark rather than wildly different).
+        1. **AI Photo Quote** — Upload photos of items on a quote; AI estimates surface area, complexity, and labor time. Available in the quote item wizard. *Availability depends on your subscription plan.* The AI is tuned for consistency — running the same photo through the wizard multiple times should produce very similar estimates each time (small differences may occur due to the visual nature of photo analysis, but the numbers will be in the same ballpark rather than wildly different). If the AI service is temporarily under high demand, the system will automatically retry (including a fallback to a secondary model) before showing an error — so a single click will usually succeed even if Anthropic's servers are briefly busy.
 
         2. **AI Inventory Assist** — AI-powered product lookup when adding or editing inventory items. Click the AI lookup button on the inventory form to auto-fill product details from a part name or description. *Availability depends on your subscription plan.*