fix(email): match IMAP card to SMTP styling + auto-detect mailbox folders

- IncomingMailConfigCard rebuilt to mirror the outgoing SMTP card: Card
  padding=md, icon inputs (Server/User/Lock), password eye toggle, stacked
  full-width fields, full-width primary Save button
- Folder is now a dropdown auto-populated by a 'Detect' button instead of a
  free-text path: backend emailIntakeService.listFolders() lists IMAP
  mailboxes (POST /admin/email/incoming-config/folders, accepts current form
  creds, masked password falls back to stored); UI auto-selects the inbox
  (special-use) folder
- en/de strings added

frontend/src/services/email.service.ts

@@ -88,6 +88,13 @@ export interface IncomingMailConfig {
   imap_folder: string;
 }
 
+export interface ImapFolder {
+  path: string;
+  name: string;
+  /** IMAP special-use flag, e.g. '\\Inbox', '\\Sent' — used to auto-select. */
+  specialUse: string | null;
+}
+
 export interface ReceivedEmail {
   id: number;
   message_id: string | null;
@@ -125,6 +132,12 @@ export const emailService = {
   async updateIncomingConfig(config: IncomingMailConfig): Promise<void> {
     await api.post('/admin/email/incoming-config', config);
   },
+  // Auto-detect mailbox folders. Sends the current form values so detection
+  // works before the config is saved (masked password falls back server-side).
+  async listIncomingFolders(config?: Partial<IncomingMailConfig>): Promise<ImapFolder[]> {
+    const response = await api.post<{ folders: ImapFolder[] }>('/admin/email/incoming-config/folders', config || {});
+    return response.data.folders;
+  },
   async listReceived(params: { page?: number; pageSize?: number } = {}): Promise<ReceivedEmailsResponse> {
     const response = await api.get<ReceivedEmailsResponse>('/admin/email/received', { params });
     return response.data;