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/pages/Classes/detail.tsx

@@ -62,7 +62,6 @@ interface ClassDetail {
   id: number;
   name: string;
   code: string;
-  departmentId: number | null;
   classType: string;
   startDate: string | null;
   endDate: string | null;

apps/admin/src/pages/Classes/index.tsx

@@ -16,7 +16,6 @@ interface ClassItem {
   id: number;
   name: string;
   code: string;
-  departmentId: number | null;
   classType: string;
   startDate: string | null;
   endDate: string | null;