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)

backend/src/services/emailProcessor.js

@@ -726,9 +726,12 @@ async function sendTemplateEmail(to, templateKey, variables) {
       throw new Error('Email configuration not found');
     }
 
-    // Determine recipient language (pass eventId if available in variables)
-    const language = await getRecipientLanguage(to, variables.eventId || null);
-    
+    // Determine recipient language. An explicit `__language` in the email data
+    // wins (CRM/billing emails set it to the customer/invoice language so a
+    // gallery event's language can't override a dunning notice — see #760);
+    // otherwise fall back to the event-first recipient resolution.
+    const language = variables.__language || await getRecipientLanguage(to, variables.eventId || null);
+
     // Process template with variables
     const { subject, htmlBody, textBody } = await processTemplate(template, variables, language);
 
@@ -783,7 +786,7 @@ async function sendTemplateEmail(to, templateKey, variables) {
 async function renderQueuedEmail(templateKey, variables = {}, to = '') {
   const template = await db('email_templates').where('template_key', templateKey).first();
   if (!template) return null;
-  const language = await getRecipientLanguage(to, variables.eventId || null);
+  const language = variables.__language || await getRecipientLanguage(to, variables.eventId || null);
   const { subject, htmlBody } = await processTemplate(template, variables, language);
   return { subject, html: htmlBody };
 }

backend/src/services/invoice/reminders.js

@@ -141,7 +141,7 @@ async function applyReminder(invoice, lineItems, level, adminId) {
   const rawDaysOverdue = Math.floor((Date.now() - new Date(invoice.due_date).getTime()) / 86400000);
   const daysOverdue = Math.max(1, rawDaysOverdue);
   const templateKey = level === 1 ? 'invoice_reminder_first' : 'invoice_reminder_second';
-  const locale = ctx.locale || invoice.language || 'de';
+  const locale = ctx.locale || customer.preferred_language || invoice.language || 'de';
   const outstandingMinor = Math.max(0, newTotal - Number(invoice.paid_amount_minor || 0));
 
   // Attach the (unchanged) original invoice PDF + the new Mahnung.
@@ -154,6 +154,8 @@ async function applyReminder(invoice, lineItems, level, adminId) {
   const { to: reminderTo, cc: reminderCc } = resolveBillingRecipients(customer, invoice.cc_pdf_email);
   try {
     await emailProcessor.queueEmail(invoice.event_id || null, reminderTo, templateKey, {
+      // Render in the customer/invoice language, not the gallery event's (#760).
+      __language: locale,
       invoice_number: invoice.invoice_number,
       customer_name: customer.display_name || customer.first_name || customer.email.split('@')[0],
       total_amount: formatMajor(invoice.total_amount_minor, invoice.currency, locale),