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/test/fixtures.ts

@@ -221,6 +221,5 @@ export const PERMISSION_NODES = [
   'log:view',
   'role:view', 'role:add', 'role:update', 'role:delete',
   'user:view', 'user:add', 'user:update', 'user:delete',
-  'department:view', 'department:add', 'department:update', 'department:delete',
   'dashboard:view',
 ] as const;