feat(users): allow role assignment for Azure AD and LDAP users

- Backend: add admin-only guard + role validation to PUT /users/{id}
- Backend: prevent admins from changing their own role
- Frontend: role toggle button (person-check / person-dash) per user row
- Frontend: admin badge green, viewer badge secondary, ldap badge blue
- i18n: add makeAdmin / makeViewer translations (de + en)

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

static/lang/en.json

@@ -191,6 +191,8 @@
     "saveBranding": "Save Branding",
     "userManagement": "User Management",
     "newUser": "New User",
+    "makeAdmin": "Promote to admin",
+    "makeViewer": "Demote to viewer",
     "thId": "ID",
     "thUsername": "Username",
     "thEmail": "Email",