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/i18n/locales/de.json

@@ -2486,6 +2486,12 @@
       "user": "Benutzername",
       "pass": "Passwort",
       "folder": "Ordner",
+      "save": "Eingangs-E-Mail-Einstellungen speichern",
+      "detectFolders": "Erkennen",
+      "folderHint": "Host, Benutzername und Passwort eingeben, dann „Erkennen“, um die Postfach-Ordner aufzulisten.",
+      "foldersDetected": "{{count}} Ordner gefunden.",
+      "noFolders": "Der Server hat keine Ordner zurückgegeben.",
+      "detectFailed": "Ordner konnten nicht erkannt werden.",
       "savedToast": "Einstellungen für eingehende E-Mails gespeichert."
     },
     "received": {

frontend/src/i18n/locales/en.json

@@ -2059,6 +2059,12 @@
       "user": "Username",
       "pass": "Password",
       "folder": "Folder",
+      "save": "Save Incoming Mail Settings",
+      "detectFolders": "Detect",
+      "folderHint": "Enter host, username and password, then Detect to list the mailbox folders.",
+      "foldersDetected": "{{count}} folders found.",
+      "noFolders": "No folders returned by the server.",
+      "detectFailed": "Could not detect folders.",
       "savedToast": "Incoming mail settings saved."
     },
     "received": {