feat(attention): unsent and unreminded invoices (P4b S5)

The books record that an invoice went out but do not chase it. U0b's
one attention engine now lists two items the sent record makes possible,
in the viewer's Needs attention and in books.start's next steps alike:

- "Invoice INV-0007 was never sent": an issued invoice (a number) still
  Open or Partial, with no send recorded, once today is past its issue
  date plus invoicing.unsent_days (default 2). A legacy invoice with no
  number is exempt, and so is a void or settled one.
- "Invoice INV-0007 is overdue: remind the customer": the invoice is
  overdue and its last send is older than invoicing.reminder_days
  (default 14). An invoice never sent gets the first item, not this one.

Both are Todo, open the invoice, and offer books.mark_sent with its
invoice and the tool's defaults. The viewer maps the new
Target::Invoice to its invoice page (one line; items still come only
from ledger-package), and the two lists cannot drift.

Tests: an_unsent_invoice_is_listed_until_a_send_clears_it (the overview
draws it; attention-unsent-invoice.png; a send clears it),
a_legacy_invoice_with_no_number_is_never_listed_as_unsent,
an_overdue_invoice_with_an_old_send_is_asked_to_remind, and
an_unsent_invoice_is_on_both_lists (viewer and books.start, step
books.mark_sent on invoice:1). The existing attention parity test stays
green.

crates/ledger-gui/src/attention.rs

@@ -67,6 +67,7 @@ fn route(t: ledger_package::Target) -> Option<Route> {
         ledger_package::Target::Assets => Route::Assets,
         ledger_package::Target::Asset(a) => Route::Asset(a),
         ledger_package::Target::Documents => Route::Documents,
+        ledger_package::Target::Invoice(i) => Route::Invoice(i),
         ledger_package::Target::Journal => Route::Journal,
         ledger_package::Target::Report(r) => Route::Reports(match r {
             ledger_package::Report::Vendor1099 => ReportKind::Vendor1099,