Fix: ManageUsers component missing useAuth() - caused blank admin page

The ManageUsers sub-component referenced user?.role for conditional
role dropdown options but didn't call useAuth() to get the user object.
This caused 'user is not defined' JS errors and blank admin pages.

Browser E2E: 20/20 tests pass.

backend/src/utils/validation.js

@@ -14,7 +14,7 @@ const registerSchema = z.object({
     .min(8, 'Password must be at least 8 characters')
     .max(128),
   name: z.string().min(1, 'Name is required').max(100).trim(),
-  role: z.enum(['employee', 'admin', 'super_admin']).default('employee'),
+  role: z.enum(['employee', 'office', 'admin', 'super_admin']).default('employee'),
 });
 
 const refreshSchema = z.object({