feat: the invoice PDF, with stamps and each payment (P4b S3)

ledger-import invoice_pdf (templates/invoice.typ): seller, bill-to,
number, dates, terms, lines, total, each settled line and the balance
due; money formatted in Rust (the template does no arithmetic), text as
data, deterministic bytes. Stamps as a rubber stamp would mark it, in
red: PAID (with the day it was paid in full: the invoice is then the
customer's receipt), PARTIALLY PAID, PAST DUE, VOID; DRAFT in gray for
S4's preview. ledger-package invoice_document builds it from the books
as of a day: the derived figures (B-38), each payment that stands on its
own line with its date, payment method and reference; re-created copies
say so (the issued copy is kept from S4 on and never changes).
books.document pdf hands it over as a download; the viewer's invoice
page saves or opens it. The owner asked for the paid stamp with the
payment's date and details (2026-10-24).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T3gXMVt7XJPi6ens1or9Dj

crates/ledger-gui/tests/invoices.rs

@@ -175,6 +175,8 @@ fn a_documents_page_shows_its_timeline() {
     );
     // Received: nothing stands.
     assert!(h.query_by_label("$0.00").is_some());
+    // An invoice can be saved as PDF (a bill can't: it is the vendor's).
+    assert!(h.query_by_label_contains("Save invoice PDF").is_some());
     // A timeline row opens its entry.
     h.get_by_label_contains(&format!("Payment (reversed later), entry #{pay},"))
         .click();