fix(email,ui): billing emails follow customer language + readable payment-check confirmation

- Billing/dunning emails no longer render in the gallery event's language.
  emailProcessor now honors an explicit `__language` in the email data
  (else falls back to the event-first recipient resolution), and the invoice
  reminder passes the customer/invoice locale (customer.preferred_language
  || invoice.language || 'de'). Fixes German customers getting English
  dunning notices. (#760)
- Payment-check confirmation card ("Action recorded") is now theme-adaptive
  (green tint + readable text on both light and dark surfaces) instead of a
  hardcoded light-green mix + dark-green title that vanished in dark mode. (#759)
- The per-customer "Preferred language" field already exists
  (CustomerDetailPage) plus the business-profile default; updated the helper
  text to note billing emails now honor it too. (#761)

frontend/src/pages/admin/CustomerDetailPage.tsx

@@ -360,7 +360,7 @@ export const CustomerDetailPage: React.FC = () => {
             </select>
             <p className="text-xs text-neutral-500 mt-1">
               {t('customers.detail.preferredLanguageHint',
-                'Drives portal UI and quote/invoice PDF locale. New customers default to the business-profile language ({{lang}}); override here per customer.',
+                'Drives portal UI, quote/invoice PDFs, and billing emails (reminders/dunning). New customers default to the business-profile language ({{lang}}); override here per customer.',
                 { lang: LOCALE_LABELS[profileDefaultLocale] || profileDefaultLocale.toUpperCase() })}
             </p>
           </div>

frontend/src/pages/public/PaymentCheckPage.tsx

@@ -373,15 +373,18 @@ const ResultBox: React.FC<{
         <div
           className="rounded-lg border p-6"
           style={{
-            borderColor: '#bbf7d0',
-            backgroundColor: 'color-mix(in srgb, #dcfce7 50%, var(--color-surface))',
+            // Theme-adaptive success card — a light-green tint on light surfaces,
+            // a dark-green tint on dark ones (was a hardcoded light-green mix +
+            // dark-green title that went unreadable in dark mode, #759).
+            borderColor: 'color-mix(in srgb, #16a34a 35%, var(--color-surface))',
+            backgroundColor: 'color-mix(in srgb, #16a34a 12%, var(--color-surface))',
           }}
         >
           <CheckCircle2 className="w-10 h-10 mb-3" style={{ color: '#16a34a' }} />
-          <h1 className="text-lg font-bold mb-1" style={{ color: '#166534' }}>
+          <h1 className="text-lg font-bold mb-1" style={{ color: 'var(--color-text)' }}>
             {t('paymentCheck.result.title', 'Action recorded')}
           </h1>
-          <p className="text-sm">
+          <p className="text-sm" style={{ color: 'var(--color-text)' }}>
             {result.applied === 'paid_full' && t('paymentCheck.result.paid',
               'Invoice {{n}} marked as paid in full.', { n: inv.invoiceNumber })}
             {result.applied === 'paid_with_skonto' && t('paymentCheck.result.paidSkonto',