feat: update frontend for four-tier role system

Add Student Admin and Teacher Admin roles to constants, types, store,
permissions, routes, and admin UI. Teacher Admin sees contests and
problemsets in sidebar; Student Admin sees only problems.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/admin/user/components/Name.vue

@@ -24,7 +24,7 @@ const isNotRegularUser = computed(
     >
       {{ getUserRole(props.user.admin_type).label }}
     </n-tag>
-    <n-tag size="small" v-if="props.user.admin_type === USER_TYPE.ADMIN">
+    <n-tag size="small" v-if="props.user.admin_type === USER_TYPE.STUDENT_ADMIN || props.user.admin_type === USER_TYPE.TEACHER_ADMIN">
       {{
         props.user.problem_permission === PROBLEM_PERMISSION.ALL
           ? "全部"