feat(email): incoming mail UI - IMAP config block + Received emails tab

Frontend for the incoming-mail feature.

- Settings -> Email: an "Incoming mail (IMAP)" block under the outgoing SMTP
  settings (same field shape: host/port/security/user/pass/folder), shown only
  when the incomingMail flag is on (IncomingMailConfigCard, self-contained
  load/save).
- A "Received emails" tab next to "Sent emails" (ReceivedEmailsPanel) listing
  the received_emails log with from/subject/received/status + attachment count
  and a link to the incoming-invoices inbox.
- `incomingMail` flag in the frontend (type + context default, standalone) +
  a Communication-section Features card.
- email.service: getIncomingConfig / updateIncomingConfig / listReceived.
- i18n: settings.features.incomingMail, email.incoming, email.received (EN+DE).

Verified: tsc --noEmit clean (0 errors); en/de JSON valid; npm run build green.

frontend/src/services/email.service.ts

@@ -79,6 +79,32 @@ export interface EmailPreview {
   body_text: string;
 }
 
+export interface IncomingMailConfig {
+  imap_host: string;
+  imap_port: number;
+  imap_secure: boolean;
+  imap_user: string;
+  imap_pass: string;
+  imap_folder: string;
+}
+
+export interface ReceivedEmail {
+  id: number;
+  message_id: string | null;
+  from_address: string | null;
+  subject: string | null;
+  received_at: string | null;
+  attachment_count: number;
+  status: string;
+  inbound_document_id: number | null;
+  error: string | null;
+}
+
+export interface ReceivedEmailsResponse {
+  items: ReceivedEmail[];
+  pagination: { page: number; pageSize: number; total: number; totalPages: number };
+}
+
 export const emailService = {
   // Get email configuration
   async getConfig(): Promise<EmailConfig> {
@@ -91,6 +117,19 @@ export const emailService = {
     await api.post('/admin/email/config', config);
   },
 
+  // Incoming mail (IMAP) configuration
+  async getIncomingConfig(): Promise<IncomingMailConfig> {
+    const response = await api.get<IncomingMailConfig>('/admin/email/incoming-config');
+    return response.data;
+  },
+  async updateIncomingConfig(config: IncomingMailConfig): Promise<void> {
+    await api.post('/admin/email/incoming-config', config);
+  },
+  async listReceived(params: { page?: number; pageSize?: number } = {}): Promise<ReceivedEmailsResponse> {
+    const response = await api.get<ReceivedEmailsResponse>('/admin/email/received', { params });
+    return response.data;
+  },
+
   // Test email configuration
   async testEmail(testEmail: string): Promise<void> {
     await api.post('/admin/email/test', { test_email: testEmail });