feat(viewer): the invoice page shows its number, terms, bill-to and PO (P4b S4.5)

The viewer reads the same invoice the server prints. The Money in list and
the page title name an invoice by its issued number (a legacy invoice keeps
"Invoice {id}"); a Details card shows the number, terms, the bill-to as it
was at issue (contact, address, phone, email), Ship-to, the customer's PO
and memo, and a bill's vendor number, day received, terms and memo. Save
invoice PDF still re-creates the current copy; when the issued copy is
filed (found through its link to the invoice, never by name) Save issued
copy and Open issued copy hand over those very bytes, checked against
their SHA-256. Nothing is written to the books.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HS8WZziDnyxunC23MUxrLp

crates/ledger-gui/tests/view_model.rs

@@ -924,3 +924,64 @@ fn a_write_off_is_flagged_for_the_cpa() {
     assert_eq!(st.status, ledger_core::DocStatus::WrittenOff);
     assert!(books.ledger.get_entry(entry).is_some());
 }
+
+/// Plan P4b S4.5: the viewer, which replays the journal itself, holds a
+/// numbered invoice exactly as the store that wrote it does: its number,
+/// terms, bill-to snapshot, the customer's PO and memo, and its lines'
+/// quantity and unit price. What the page shows is read from those, so
+/// the viewer and the server cannot disagree about them.
+#[test]
+fn the_viewer_holds_a_numbered_invoice_as_the_store_does() {
+    use ledger_core::arap::InvoiceFields;
+    use ledger_core::{PartySnapshot, Terms};
+    let d = dir("numbered-parity");
+    let today = NaiveDate::from_ymd_opt(2026, 3, 15).unwrap();
+    let mut s = Store::create(&d.join("books-data"), Currency::USD).unwrap();
+    s.add_account("1000", "Checking", AccountType::Asset, None, false)
+        .unwrap();
+    let cust = s.add_customer("Harbor Coffee").unwrap();
+    let inv = s
+        .create_invoice_with(
+            cust,
+            NaiveDate::from_ymd_opt(2026, 3, 1).unwrap(),
+            NaiveDate::from_ymd_opt(2026, 3, 31).unwrap(),
+            vec![DocumentLine {
+                description: "Beans".into(),
+                amount: Decimal::new(9_999, 2),
+                class_id: None,
+                account: None,
+                quantity: Some(Decimal::new(3, 0)),
+                unit_price: Some(Decimal::new(3_333, 2)),
+            }],
+            InvoiceFields {
+                number: Some("INV-0007".into()),
+                terms: Some(Terms::parse("2/10 net 30").unwrap()),
+                bill_to: Some(PartySnapshot {
+                    name: "Harbor Coffee Roasters, Inc.".into(),
+                    address: Some("455 Wharf Street".into()),
+                    email: Some("ap@harbor.example".into()),
+                    phone: None,
+                    contact: Some("Ana Reyes".into()),
+                    billing_address: None,
+                    shipping_address: Some("Dock 4".into()),
+                    billing_email: None,
+                }),
+                seller: None,
+                customer_ref: Some("PO-4471".into()),
+                memo: Some("Thanks".into()),
+            },
+        )
+        .unwrap();
+    let stored = s.ledger().get_invoice(inv).unwrap().clone();
+    drop(s);
+    let books = data::load_once(&Source {
+        data_dir: d.join("books-data"),
+        today: Some(today),
+    })
+    .unwrap();
+    let shown = books.ledger.get_invoice(inv).unwrap();
+    assert_eq!(shown, &stored);
+    assert_eq!(shown.number.as_deref(), Some("INV-0007"));
+    assert_eq!(shown.terms.as_ref().unwrap().describe(), "2/10 net 30");
+    assert_eq!(shown.lines[0].quantity, Some(Decimal::new(3, 0)));
+}