feat: add invoice row details, tax summary fields, and fix XML element ordering

[samupl]

• Fix: Nazwa element placement — Moved buyer name (Nazwa) inside DaneIdentyfikacyjne where the FA(3) schema requires it, instead of directly under Podmiot2
• Fix: XML element ordering — Moved Adres before Nazwa in Podmiot2, and P_13/P_14 tax summary before P_15 in Fa
• Add: Invoice row fields — InvoiceRow now supports unit_of_measure (P_8A), quantity (P_8B), unit_net_price (P_9A), net_value (P_11), and delivery_date (P_6A)
• Add: Tax rate type safety — Replaced int tax field with TaxRate literal type covering all 14 valid TStawkaPodatku enum values. Added constants (TAX_23, TAX_0_WDT, TAX_NP_I, etc.)
• Add: OSS/IOSS support — InvoiceRow.tax_oss field (P_12_XII) for EU consumer VAT rates not in the standard enum (e.g. 21%, 19%)
• Add: Tax summary — TaxSummary model with all P_13_/P_14_ fields for net/VAT totals per rate group, added as optional field on InvoiceData