feat: full invoice editing before payment, NAS cleanup on date change

- Invoice edit mode now uses the same form as create mode (all fields editable)
- Bank account pre-selected by matching IBAN/account number
- Invoice number read-only in edit mode
- Paid invoices remain read-only
- NAS: old PDF deleted when invoice date changes to different month
- Buttons: Zobrazit fakturu, Uložit, Smazat + status transitions

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/routes/admin/invoices-pdf.ts

@@ -1008,6 +1008,7 @@ ${indentCSS}
           ? new Date(invoice.issue_date)
           : new Date();
         const saveMode = query.save === "1";
+        nasFinancialsManager.cleanIssuedInvoice(invoice.invoice_number!);
         const pdfPromise = htmlToPdf(html)
           .then((pdfBuffer) => {
             nasFinancialsManager.saveIssuedInvoicePdf(