Phase F: Customer/Vendor Statements, Payment Terms Parser, Tax Rates

F1: GetCustomerStatementAsync/GetVendorStatementAsync on IFinancialReportService;
    StatementLineDto; CustomerStatementDto/VendorStatementDto; Statement action on
    CustomersController + VendorsController; Statement views + PDF download via
    StatementPdfHelper (QuestPDF); Statement button on Customer/Vendor Details pages.

F2: PaymentTermsParser static helper (CalculateDueDate, ParseEarlyPaymentDiscount);
    EarlyPaymentDiscountPercent/Days on Invoice entity; GetCustomerPaymentTerms AJAX
    endpoint on InvoicesController auto-populates Terms + due date on customer select;
    early payment discount notice on Invoice Create.

F3: TaxRate entity (Name/Rate/State/IsDefault/IsActive, tenant-filtered);
    IUnitOfWork.TaxRates + UnitOfWork + ApplicationDbContext; TaxRatesController
    (Index/Create/Edit/Delete/ToggleActive, CompanyAdminOnly); GetTaxRateForCustomer
    AJAX endpoint; Tax Rates in Settings gear menu.

Also fixes AddVendorCredits migration: VendorCreditApplications FKs changed from
CASCADE to NoAction to resolve SQL Server error 1785 (multiple cascade paths).
Migration: AddPaymentTermsAndTaxRates applied locally; 200/200 unit tests pass.

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

src/PowderCoating.Application/Interfaces/IFinancialReportService.cs

@@ -35,4 +35,10 @@ public interface IFinancialReportService
 
     /// <summary>Looks up the accounting method configured for the given company. Returns Accrual if not found.</summary>
     Task<AccountingMethod> GetCompanyAccountingMethodAsync(int companyId);
+
+    /// <summary>Returns a dated activity statement for a customer showing opening balance, all transactions in the period, and closing balance.</summary>
+    Task<CustomerStatementDto> GetCustomerStatementAsync(int companyId, int customerId, DateTime from, DateTime to);
+
+    /// <summary>Returns a dated activity statement for a vendor showing opening balance, all transactions in the period, and closing balance.</summary>
+    Task<VendorStatementDto> GetVendorStatementAsync(int companyId, int vendorId, DateTime from, DateTime to);
 }