feat(email): per-variant paid/unpaid body templates in settings

The previous single body field could not distinguish 'use the default' from
'save the pre-filled default', so the first settings save locked the hint in
as a custom override for ALL orders — the paid/unpaid default logic never ran.

Give each payment state its own editable body field (emailBodyHtmlDePaid /
emailBodyHtmlEnPaid) with its own empty-fallback. A body submitted unchanged
from its built-in default is stored as empty, so 'use default' survives saves
and future default updates propagate.

app/services/invoice/email.server.ts

@@ -91,26 +91,23 @@ export async function sendInvoiceEmail(
     (language === "en" ? DEFAULT_EMAIL_SUBJECT_EN : DEFAULT_EMAIL_SUBJECT_DE);
   const subject = renderTemplate(customSubject, vars, { html: false });
 
-  // Pick the default body variant for the invoice's payment state: orders that
-  // are already settled (paid / refunded / voided) get a body WITHOUT the
+  // Pick the body variant for the invoice's payment state: orders that are
+  // already settled (paid / refunded / voided) get a body WITHOUT the
   // bank-transfer reference hint; outstanding orders (unpaid / partial) keep
-  // it. Legacy rows have an empty paymentStatus → treated as unpaid.
-  // A merchant-supplied custom template always wins over the defaults.
+  // it. Legacy invoice rows have an empty paymentStatus → treated as unpaid.
+  // Each variant has its own merchant-customisable field; empty falls back to
+  // the built-in default for that variant.
   const isPaid =
     invoice.paymentStatus === "paid" ||
     invoice.paymentStatus === "refunded" ||
     invoice.paymentStatus === "voided";
-  const defaultBody =
-    language === "en"
-      ? isPaid
-        ? DEFAULT_EMAIL_BODY_EN_PAID
-        : DEFAULT_EMAIL_BODY_EN_UNPAID
-      : isPaid
-        ? DEFAULT_EMAIL_BODY_DE_PAID
-        : DEFAULT_EMAIL_BODY_DE_UNPAID;
-  const customBodyHtml =
-    (language === "en" ? settings.emailBodyHtmlEn : settings.emailBodyHtmlDe) ||
-    defaultBody;
+  const customBodyHtml = isPaid
+    ? language === "en"
+      ? settings.emailBodyHtmlEnPaid || DEFAULT_EMAIL_BODY_EN_PAID
+      : settings.emailBodyHtmlDePaid || DEFAULT_EMAIL_BODY_DE_PAID
+    : language === "en"
+      ? settings.emailBodyHtmlEn || DEFAULT_EMAIL_BODY_EN_UNPAID
+      : settings.emailBodyHtmlDe || DEFAULT_EMAIL_BODY_DE_UNPAID;
   const body = renderHtmlBody(renderTemplate(customBodyHtml, vars));
 
   // If the rendered body references the inline logo, attach it.