Add invoice duplication flow (list/detail → new invoice prefill)

[capaj]

Motivation

• Provide a quick way to duplicate an existing invoice from the invoice list or detail view and open a pre-filled new invoice form.
• Ensure duplicated invoices use the next invoice number and shift date fields into the current month to avoid stale dates.

Description

• Added a "Duplikovat" menu item to IssuedInvoiceTable that links to /new-invoice?duplicateFrom=<id> and imported the Copy icon.
• Added a "Duplikovat" button to InvoiceDetailPage that navigates to the same duplicate URL and imported the Copy icon.
• In NewInvoicePage, read the duplicateFrom query parameter with useSearchParams, fetch the source invoice via trpcClient.invoices.getById (enabled only when the id is present), and reset the form when the invoice is available to pre-fill fields and items while keeping number set to the computed next invoice number.
• Implemented date-shifting logic using djs so issued_on and taxable_fulfillment_due are moved to the current month while keeping a valid day-of-month, and mapped invoice items into the new form schema.

Testing

• Ran TypeScript type-check (tsc --noEmit) which passed, and no new automated unit tests were added for this change.

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Codex Task