feat: 新增备课模块并修复全模块 P0/P1/P2 缺陷

主要变更:

- 新增 lesson-preparation 模块: 备课编辑器、节点编辑、AI 建议、知识点选择、版本历史、作业发布

- 新增 shared 通用组件: charts/question-bank-filters/schedule-list/ui (chip-nav/filter-bar/page-header/stat-card/stat-item)

- 新增 student/admin 端 loading.tsx 与 error.tsx, 优化加载与错误态体验

- 新增 teacher/lesson-plans 页面 (列表/新建/编辑)

- 新增 drizzle 迁移 0002_tiny_lionheart 及 snapshot

- 新增 textbooks/schema.ts 与 exams/utils/normalize-structure.ts

- 修复 Tiptap v3 SSR hydration 崩溃 (rich-text-block immediatelyRender: false)

- 重构多模块 data-access/actions/组件, 修复权限校验与类型规范

- 同步架构文档 004/005 反映新增模块、导出、依赖关系

- 归档 bugs/* 测试报告与 e2e 测试脚本 (admin/parent/student/teacher web_test)

src/app/(dashboard)/not-found.tsx

@@ -1,6 +1,7 @@
 import Link from "next/link"
 import { FileQuestion } from "lucide-react"
 
+import { Button } from "@/shared/components/ui/button"
 import { EmptyState } from "@/shared/components/ui/empty-state"
 
 export default function NotFound() {
@@ -12,12 +13,11 @@ export default function NotFound() {
         description="The page you are looking for does not exist or has been moved."
         className="border-none shadow-none h-auto"
       />
-      <Link
-        href="/dashboard"
-        className="bg-primary text-primary-foreground hover:bg-primary/90 inline-flex h-9 items-center justify-center rounded-md px-4 text-sm font-medium transition-colors"
-      >
-        Return to Dashboard
-      </Link>
+      <Button asChild>
+        <Link href="/dashboard">
+          Return to Dashboard
+        </Link>
+      </Button>
     </div>
   )
 }