fix(messages): PR #769 nits — server-side search, bare-email recipient, DE i18n

- Search now hits the backend (debounced) so results aren't truncated to the
  first loaded page: /received gains a `q` filter (sender/subject); the frontend
  passes the debounced term to every list query. The instant client-side filter
  stays for responsiveness.
- Reply/compose recipient extracts the bare address from a "Name <addr>" From
  header (extractEmail) — also used for the customer-lookup key.
- Added the full de + en `messages.*` and `email.customerMailbox.*` translation
  namespaces (were English inline-fallbacks only). Swiss-German spelling.

frontend/src/services/email.service.ts

@@ -220,7 +220,7 @@ export const emailService = {
     const response = await api.post<ImapPollResult>('/admin/email/incoming-config/poll', {});
     return response.data;
   },
-  async listReceived(params: { page?: number; pageSize?: number; account?: string; state?: 'active' | 'archived' | 'deleted' } = {}): Promise<ReceivedEmailsResponse> {
+  async listReceived(params: { page?: number; pageSize?: number; account?: string; state?: 'active' | 'archived' | 'deleted'; q?: string } = {}): Promise<ReceivedEmailsResponse> {
     const response = await api.get<ReceivedEmailsResponse>('/admin/email/received', { params });
     return response.data;
   },