feat(helpdesk): add user-select to schema editor field type dropdown

Adds 'user-select' to FIELD_TYPES so it appears in the schema editor
dropdown as 'Benutzerauswahl' / 'User selection'. Preview renders a
placeholder select. Translations added to both i18n files.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

modules/helpdesk/i18n/default_de.json

@@ -403,6 +403,7 @@
   "Date": "Datum",
   "Checkbox": "Checkbox",
   "Select": "Auswahl",
+  "User selection": "Benutzerauswahl",
   "Text content": "Textinhalt",
   "Maximum %d fields allowed": "Maximal %d Felder erlaubt",
   "Field %d: unknown type \"%s\"": "Feld %d: unbekannter Typ \"%s\"",