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/App.tsx

@@ -28,7 +28,6 @@ import PermissionsPage from './pages/Permissions';
 import AttendancePage from './pages/Attendance';
 import TeacherWorkspacePage from './pages/TeacherWorkspace';
 import NotificationsPage from './pages/Notifications';
-import DepartmentsPage from './pages/Departments';
 import IntegrationConfigPage from './pages/IntegrationConfig';
 import PermissionRoute from './components/PermissionRoute';
 
@@ -255,14 +254,6 @@ const App: React.FC = () => {
 
               <Route path="notifications" element={<NotificationsPage />} />
 
-              <Route
-                path="departments"
-                element={
-                  <PermissionRoute permission="department:view">
-                    <DepartmentsPage />
-                  </PermissionRoute>
-                }
-              />
 
               <Route
                 path="integration-config"