feat(teacher): 题库模块（QuestionBank）

工作内容

- 新增 /teacher/questions 页面，支持 Suspense/空状态/加载态

- 题库 CRUD Server Actions：创建/更新/递归删除子题，变更后 revalidatePath

- getQuestions 支持 q/type/difficulty/knowledgePointId 筛选与分页返回 meta

- UI：表格列/筛选器/创建编辑弹窗，content JSON 兼容组卷

- 更新中文设计文档：docs/design/004_question_bank_implementation.md

src/app/(dashboard)/teacher/questions/loading.tsx

@@ -0,0 +1,30 @@
+import { Skeleton } from "@/shared/components/ui/skeleton"
+
+export default function Loading() {
+  return (
+    <div className="flex h-full flex-col space-y-8 p-8">
+      <div className="flex flex-col justify-between space-y-4 md:flex-row md:items-center md:space-y-0">
+        <div className="space-y-2">
+          <Skeleton className="h-7 w-[200px]" />
+          <Skeleton className="h-4 w-[420px]" />
+        </div>
+        <Skeleton className="h-9 w-[140px]" />
+      </div>
+
+      <div className="space-y-4">
+        <Skeleton className="h-10 w-full" />
+        <div className="rounded-md border bg-card">
+          <div className="p-4">
+            <Skeleton className="h-8 w-full" />
+          </div>
+          <div className="space-y-2 p-4 pt-0">
+            {Array.from({ length: 6 }).map((_, idx) => (
+              <Skeleton key={idx} className="h-10 w-full" />
+            ))}
+          </div>
+        </div>
+      </div>
+    </div>
+  )
+}
+