feat(admin): add dedicated routes for admin user management (#37)

- New GET /admin/users/new page: create user with all profile fields
  (login, password, firstName, lastName, birthDate, email, contact, groups)
- New GET /admin/users/[id] page: edit user profile, groups, and
  optional password change without requiring current password
- New PUT /api/users/{id} backend endpoint (ADMIN_USER permission)
  with AdminUpdateUserRequest DTO for admin-override user updates
- Refactored admin users tab: replaced inline editing with edit links
  to dedicated routes; create button now links to /admin/users/new
- Extended CreateUserRequest with profile fields so new users can be
  created with full profile data in a single request
- Added 28 component tests across 3 new spec files (TDD)

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

frontend/messages/de.json

@@ -159,6 +159,14 @@
 	"admin_section_new_group": "Neue Gruppe anlegen",
 	"admin_group_name_placeholder": "Gruppenname (z.B. Editoren)",
 	"admin_user_delete_confirm": "Benutzer {username} wirklich löschen?",
+	"admin_btn_new_user": "Neuer Benutzer",
+	"admin_user_new_heading": "Neuen Benutzer anlegen",
+	"admin_user_edit_heading": "Benutzer bearbeiten: {username}",
+	"admin_user_created": "Benutzer wurde erstellt.",
+	"admin_user_updated": "Änderungen gespeichert.",
+	"admin_col_full_name": "Name",
+	"admin_label_new_password_optional": "Neues Passwort (optional)",
+	"admin_label_initial_password": "Passwort",
 	"doc_file_error_preview": "Vorschau konnte nicht geladen werden.",
 	"doc_download_title": "Herunterladen",
 	"doc_tag_filter_title": "Nach {name} filtern",