feat(123): receipts export xlsx VAT breakdown

- AccountingController::export(): new headers (Numer | Data wystawienia | Kwota brutto | Kwota netto | Stawka VAT | Kwota VAT), removed Data sprzedazy/Konfiguracja/Nr zamowienia/Nr referencyjny
- buildVatBreakdown() helper groups items_json by vat rate, emits one XLSX row per (receipt x rate); legacy receipts (no `vat` in snapshot) fallback to net=brutto/1.23
- ReceiptService::buildItemsSnapshot(): writes `vat` per item from order_items.tax_rate (fallback 23.0); shipping cost item gets vat=23.0
- RECEIPT-NET-FIX deferred (.paul/codebase/todo.md): ReceiptService::issue() still saves total_net=total_gross

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.paul/ROADMAP.md

@@ -22,6 +22,7 @@ Wystawianie faktur dla klientow z NIP poprzez integracje z Fakturownia (app.fakt
 | 120 | Alert Component Unification | 1/1 | Complete (2026-05-12; CSS rebuilt; smoke tests pending operator) |
 | 121 | SMSPLANET Conversation + Notifications | 1/1 | Complete (2026-05-12; live SMS/browser smoke pending operator) |
 | 122 | SMSPLANET Default SMS Footer | 1/1 | Complete (2026-05-12; live SMS smoke + over-limit UI test pending operator) |
+| 123 | Receipts Export VAT Breakdown | 1/1 | Complete (2026-05-12; manual XLSX smoke pending operator) |
 
 Planowane kolejne fazy v3.7 (kandydaci, do rozplanowania):
 - Eksport XLSX listy wystawionych faktur (analogicznie do paragonow)
@@ -504,4 +505,4 @@ Archive: `.paul/milestones/v0.1-ROADMAP.md`
 
 ---
 *Roadmap created: 2026-03-12*
-*Last updated: 2026-05-12 - Phase 118 + 121 + 122 committed; UNIFY closed for all three*
+*Last updated: 2026-05-12 - Phase 123 UNIFY closed*