Admin user management: backend API and frontend UI.

- Add admin-only users API: list/create/delete (prevent self-delete and last-admin deletion).
- Include is_admin in auth responses.
- Frontend: /admin/users page with table, selection, remove, Add User modal.
- Show “Manage users” in user menu for admins and optional sidebar link.
- Add i18n strings for admin UI.
- Enhance create user script to grant admin via optional third arg.
- Minor: set dev bootstrap user as admin in start script.

public/locales/en/translation.json

@@ -916,4 +916,19 @@
     "license": "Licensed for personal use",
     "builtBy": "Built by"
   }
+  ,
+  "admin": {
+    "manageUsers": "Manage users",
+    "userManagement": "User Management",
+    "addUser": "Add user",
+    "remove": "Remove",
+    "email": "Email",
+    "created": "Created",
+    "role": "Role",
+    "loadingUsers": "Loading users...",
+    "noUsers": "No users",
+    "admin": "admin",
+    "user": "user",
+    "password": "Password"
+  }
 }