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/pages/helpdesk/software-products/_form.phtml

@@ -53,6 +53,7 @@ $handoverSchemaFields = is_array($handoverSchemaFields ?? null) ? $handoverSchem
               'type_date' => t('Date'),
               'type_checkbox' => t('Checkbox'),
               'type_select' => t('Select'),
+              'type_user-select' => t('User selection'),
               'text_content' => t('Text content'),
               'field' => t('Field'),
               'preview' => t('Preview'),