Fix isPaidImmediately's exact-match regression, add unpaid Vorkasse instructions

isPaidImmediately() matched paymentMethodTitle with an exact !== check
against the literal "Überweisung" — the consuming shop renamed its
Vorkasse row to "Überweisung (Vorkasse)" the same day, which would have
silently made every unpaid Vorkasse invoice show "✓ Bereits beglichen".
Changed to startsWith("Überweisung") — same design, tolerant of a
suffix qualifier.

Also adds an explicit instruction for the unpaid case (previously the
absence of the paid confirmation was the only signal, no actual text):
pay to the listed bank details, order processed after payment received
(~1-2 business days).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -29,6 +29,8 @@ One canonical implementation, consumed by both repos, makes this class of drift
 
 **2026-07-24, Kleinunternehmerregelung (§19 UStG).** `InvoiceOrder`/`CorrectionInvoiceOrder` gained optional `kleinunternehmer`, a third order-level VAT treatment alongside `vatExempt` — see "Kleinunternehmerregelung" below. Takes precedence over `vatExempt` wherever both would otherwise apply (a Kleinunternehmer never charges VAT to begin with, so there's nothing left to "exempt" via the separate intra-community rule).
 
+**2026-07-25, Vorkasse fixes (v0.2.3).** `isPaidImmediately()` matched `paymentMethodTitle` with an exact `!==` check against the literal string `"Überweisung"` — the consuming shop renamed its Vorkasse row to `"Überweisung (Vorkasse)"` the same day (to make room for a possible future *automated* bank-transfer method, e.g. "Sofortüberweisung", routed through a real payment gateway), which would have silently made every unpaid Vorkasse invoice show "✓ Bereits beglichen". Changed to `startsWith("Überweisung")` — same "only Überweisung is the named exception" design, tolerant of a suffix qualifier on that one title. Also added an explicit instruction in the unpaid case (previously the *absence* of the paid confirmation was the only signal, no actual text): "Bitte überweisen Sie den Rechnungsbetrag unter Angabe der Bestellnummer … auf die unten stehende Bankverbindung. Die Bestellung wird nach Zahlungseingang bearbeitet (in der Regel innerhalb von 1–2 Werktagen)." — same plain-text treatment as the paid confirmation (`unpaidNoticeText`, muted rather than green — an unpaid Vorkasse invoice isn't a problem, just an expected pending state).
+
 ## How this is consumed
 
 Not published to npm — installed as a git dependency: