feat(admin): remove all department/campus scoping from frontend

- Delete CampusSwitcher component and useCampus hook
- Delete Departments page and its route
- Remove campus header interceptor from API client
- Remove departmentId from Class interfaces
- Remove campusLocation from student profile
- Remove department permissions from test fixtures
- Remove currentCampusId from test cleanup

TypeScript compiles clean.

apps/admin/src/api/index.ts

@@ -10,10 +10,6 @@ instance.interceptors.request.use((config) => {
   if (token) {
     config.headers.Authorization = `Bearer ${token}`;
   }
-  const campusId = localStorage.getItem('currentCampusId');
-  if (campusId) {
-    config.headers['X-Campus-Id'] = campusId;
-  }
   return config;
 });