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/hooks/useCampus.ts

@@ -1,40 +0,0 @@
-import { useState, useEffect, useCallback } from 'react';
-import api from '../api';
-
-interface Department {
-  id: number;
-  name: string;
-  type: string;
-  parentId: number | null;
-}
-
-export function useCampus() {
-  const [campuses, setCampuses] = useState<Department[]>([]);
-  const [currentId, setCurrentId] = useState<string>(
-    () => localStorage.getItem('currentCampusId') || ''
-  );
-  const [loading, setLoading] = useState(true);
-
-  const fetchCampuses = useCallback(async () => {
-    try {
-      const data = await api.get('/departments') as unknown as Department[];
-      const campusList = data.filter((d) => d.type === 'campus');
-      setCampuses(campusList);
-      if (!currentId && campusList.length > 0) {
-        setCurrentId(String(campusList[0].id));
-        localStorage.setItem('currentCampusId', String(campusList[0].id));
-      }
-    } catch { /* ignore */ }
-    finally { setLoading(false); }
-  }, [currentId]);
-
-  useEffect(() => { fetchCampuses(); }, []);
-
-  const switchCampus = useCallback((id: string) => {
-    setCurrentId(id);
-    localStorage.setItem('currentCampusId', id);
-    window.dispatchEvent(new CustomEvent('campus-changed', { detail: id }));
-  }, []);
-
-  return { campuses, currentId, switchCampus, loading };
-}