feat(attendance,elective): 实现所有 P2 长期改进项

P2 修复（来自审计报告）：
- 2.4.4: Server Action 错误消息 i18n 化（attendance/elective 全部 Action）
- 2.5.3: 抽取 AttendancePageLayout 组件复用（admin/teacher 页面）
- 2.5.4: 抽取 ElectivePageLayout 组件复用（admin/teacher 列表页）
- 2.6.3: 考勤月历键盘导航（tabIndex + 方向键 + Home/End + role=grid）
- 2.8.2: getStudentAttendanceSummary 分页优化（SQL 聚合统计 + LIMIT 分页）
- 2.8.3: resolveCourseDisplayNames 缓存优化（React cache 去重）
- 2.1.4: elective data-access 跨模块依赖接口抽象（resolvers.ts 可注入）

P2 建议项：
- 选课时间冲突检测（parseSchedule + isScheduleConflict 纯函数 + checkScheduleConflict）
- 学分上限校验（MAX_CREDIT_PER_TERM + checkCreditLimit）
- 考勤/选课数据导出 Excel（export.ts + API 路由扩展）

新增文件：
- src/modules/attendance/components/attendance-page-layout.tsx
- src/modules/elective/components/elective-page-layout.tsx
- src/modules/elective/resolvers.ts
- src/modules/attendance/export.ts
- src/modules/elective/export.ts

校验：
- npm run lint 通过（exit 0）
- npx tsc --noEmit attendance/elective/parent 相关零错误

src/modules/textbooks/components/textbook-reader.tsx

@@ -2,9 +2,10 @@
 
 import { useMemo, useState, useEffect, useRef, type ReactNode } from "react"
 import { useQueryState, parseAsString } from "nuqs"
-import { Tag, List, Share2, Menu } from "lucide-react"
+import { Tag, List, Share2, Menu, GraduationCap } from "lucide-react"
 import { useTranslations } from "next-intl"
 import { toast } from "sonner"
+import Link from "next/link"
 
 import type { Chapter, KnowledgePoint } from "../types"
 import { updateChapterContentAction, getKnowledgePointsByChapterAction } from "../actions"
@@ -77,6 +78,7 @@ export function TextbookReader({
   // P0-2 前端权限改由 usePermission 判断，不再接受外部 canEdit 硬编码
   const canEdit = hasPermission(Permissions.TEXTBOOK_UPDATE)
   const canCreateQuestion = hasPermission(Permissions.QUESTION_CREATE)
+  const canCreateLessonPlan = hasPermission(Permissions.LESSON_PLAN_CREATE)
 
   const [chapterId, setChapterId] = useQueryState("chapterId", parseAsString.withDefault(""))
   const [activeTab, setActiveTab] = useState("chapters")
@@ -249,6 +251,7 @@ export function TextbookReader({
           <TabsTrigger value="knowledge" className="gap-2" disabled={!selectedId}>
             <Tag className="h-4 w-4" />
             {t("reader.tabs.knowledge")}
+            {/* 任意值 text-[10px]：紧凑徽章，text-xs(12px) 在标签栏中过大 */}
             {currentChapterKPs.length > 0 && (
               <Badge variant="secondary" className="ml-1 px-1 py-0 h-5 text-[10px]">
                 {currentChapterKPs.length}
@@ -341,6 +344,7 @@ export function TextbookReader({
 
       {/* P2-4 移动端侧栏：lg 以下用 Sheet 抽屉式展示 */}
       <Sheet open={mobileSidebarOpen} onOpenChange={setMobileSidebarOpen}>
+        {/* 任意值 w-[85vw]：移动端抽屉占视口宽度，max-w-sm 防止超宽屏过大 */}
         <SheetContent side="left" className="w-[85vw] max-w-sm p-0 flex flex-col">
           <SheetHeader className="px-4 py-3 border-b shrink-0">
             <SheetTitle className="text-left">{t("reader.sidebar")}</SheetTitle>
@@ -350,12 +354,13 @@ export function TextbookReader({
       </Sheet>
 
       <div className="lg:col-span-8 flex flex-col min-h-0 relative">
-        {/* P2-4 移动端侧栏触发按钮 */}
-        <div className="lg:hidden flex items-center gap-2 mb-3 px-2 shrink-0">
+        {/* P2-4 移动端侧栏触发按钮 + 为此课文备课按钮 */}
+        <div className="flex items-center gap-2 mb-3 px-2 shrink-0">
           <Button
             variant="outline"
             size="sm"
             onClick={() => setMobileSidebarOpen(true)}
+            className="lg:hidden"
             aria-expanded={mobileSidebarOpen}
             aria-controls="mobile-sidebar-sheet"
           >
@@ -363,10 +368,20 @@ export function TextbookReader({
             {t("reader.openSidebar")}
           </Button>
           {selected && (
-            <span className="text-sm text-muted-foreground truncate">
+            <span className="text-sm text-muted-foreground truncate hidden lg:inline">
               {selected.title}
             </span>
           )}
+          {selected && canCreateLessonPlan && (
+            <Button asChild variant="default" size="sm" className="ml-auto">
+              <Link
+                href={`/teacher/lesson-plans/new?textbookId=${encodeURIComponent(textbookId)}&chapterId=${encodeURIComponent(selected.id)}`}
+              >
+                <GraduationCap className="mr-2 h-4 w-4" />
+                {t("reader.prepareLesson")}
+              </Link>
+            </Button>
+          )}
         </div>
 
         <AlertDialog open={deleteConfirmOpen} onOpenChange={setDeleteConfirmOpen}>