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/layouts/MainLayout.tsx

@@ -29,7 +29,6 @@ import {
 } from '@ant-design/icons';
 import { usePermission } from '../hooks/usePermission';
 import NotificationBell from '../components/NotificationBell';
-import CampusSwitcher from '../components/CampusSwitcher';
 
 const { Header, Sider, Content } = Layout;
 
@@ -109,7 +108,6 @@ const allMenuItems: MenuItemType[] = [
     label: '系统管理',
     permission: 'log:view',
     children: [
-      { key: '/departments', icon: <HomeOutlined />, label: '校区/部门', permission: 'department:view' },
       { key: '/notifications', icon: <BellOutlined />, label: '通知中心', permission: 'notification:view' },
       { key: '/operation-logs', icon: <AuditOutlined />, label: '操作日志', permission: 'log:view' },
       { key: '/roles', icon: <SafetyOutlined />, label: '角色管理', permission: 'role:view' },
@@ -289,7 +287,6 @@ const MainLayout: React.FC = () => {
             }
             onClick={() => (isMobile || isTablet ? setDrawerOpen(true) : setCollapsed(!collapsed))}
           />
-          {isDesktop && <CampusSwitcher />}
           <div style={{ display: 'flex', alignItems: 'center', gap: 16 }}>
             <NotificationBell />
             <Dropdown