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>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@einfach-produktiv/invoicing",
-  "version": "0.2.2",
+  "version": "0.2.3",
   "private": true,
   "description": "Shared invoice / correction-invoice (Stornorechnung, Gutschrift) PDF generation and VAT-breakdown math, consumed as a git dependency by both the einfach-produktiv frontend and the payload backend — not published to npm.",
   "type": "module",