Attach the invoice PDF to the payment-method-switched email, brand-tone copy

buildInvoiceAttachment() extracted out of sendOrderConfirmationEmail() so
sendPaymentSwitchedEmail() can reuse the same invoice-PDF generation
instead of duplicating it — the invoiceNumber stays the same, but
paymentMethodTitle now reflects the confirmed instrument, which flips
the PDF's own paid/unpaid display, so re-attaching a fresh copy matters
here even though nothing else about the order changed.

Default fallback copy replaced with brand-toned wording matching the
existing order-confirmation fallback's voice, instead of a flat
system-notice tone.

README.md

@@ -2104,9 +2104,21 @@ switching an existing Überweisung order to Stripe, not a fresh gated
 checkout — see the Payload backend's own README on
 `switchPaymentToStripeEndpoint`). Same shape as the order-status emails
 (icon + admin-editable heading/bodyText + "Bestellung ansehen" button via
-`renderOrderStatusHtml`), no item table or invoice re-attachment — the
-invoice itself didn't change, only how it got paid, and the customer
-already received the full order-confirmation email once already. New
+`renderOrderStatusHtml`), no item table, but **does** re-attach a freshly
+generated invoice PDF — same `invoiceNumber` as always (never re-issued
+for a switch), but `paymentMethodTitle` now reflects the actually-
+confirmed instrument, which changes `@einfach-produktiv/invoicing`'s own
+`isPaidImmediately()` check from the Vorkasse notice to "✓ Bereits
+beglichen", so the customer's copy of "their invoice" should reflect
+that. `buildInvoiceAttachment()` was extracted out of
+`sendOrderConfirmationEmail()` so both senders share the exact same PDF-
+generation call instead of duplicating that ~40-line object literal.
+Default fallback copy (used whenever no admin template is saved/active)
+matches the existing brand voice (`sendOrderConfirmationEmail`'s own
+"Bestellt! Deine Ruhe kann kommen 🎉" fallback) rather than a flat
+system-notice tone: *"Erledigt! Deine Zahlung ist da 🎉" / "Deine Zahlung
+ist gerade bei uns eingetrudelt — ab jetzt läuft alles automatisch
+weiter, du musst dich um nichts mehr kümmern."* New
 `payment-method-switched` `EmailTemplateType`, added to
 `/email-preview`'s valid types too.