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/i18n/locales/de.json

@@ -4074,7 +4074,7 @@
       "noEvents": "Noch keinem Event zugewiesen. Fügen Sie diesen Kunden über das Event-Formular hinzu.",
       "email": "E-Mail",
       "preferredLanguage": "Bevorzugte Sprache",
-      "preferredLanguageHint": "Steuert die Portal-Sprache sowie die Sprache von Angebots- und Rechnungs-PDFs. Neue Kunden erben standardmässig die Sprache aus dem Geschäftsprofil ({{lang}}); hier kann pro Kunde überschrieben werden.",
+      "preferredLanguageHint": "Steuert die Portal-Sprache, Angebots-/Rechnungs-PDFs sowie Rechnungs-E-Mails (Erinnerungen/Mahnungen). Neue Kunden erben standardmässig die Sprache aus dem Geschäftsprofil ({{lang}}); hier kann pro Kunde überschrieben werden.",
       "salutation": "Anrede",
       "salutationNone": "—",
       "firstName": "Vorname",

frontend/src/i18n/locales/en.json

@@ -4074,7 +4074,7 @@
       "noEvents": "Not assigned to any events yet. Add this customer to an event from the event form.",
       "email": "Email",
       "preferredLanguage": "Preferred language",
-      "preferredLanguageHint": "Drives portal UI and quote/invoice PDF locale. New customers default to the business-profile language ({{lang}}); override here per customer.",
+      "preferredLanguageHint": "Drives portal UI, quote/invoice PDFs, and billing emails (reminders/dunning). New customers default to the business-profile language ({{lang}}); override here per customer.",
       "salutation": "Salutation",
       "salutationNone": "—",
       "firstName": "First name",