fix(teacher): 统一详情页返回路径与中英文文案 (P1-3+P2-1)

P1-3: empty-state 默认按钮 variant 改为 outline 并新增 variant prop；button.tsx 导出 ButtonProps；统一 5 个详情页返回路径为 ghost+ArrowLeft+文字标签；course-plan-detail raw a 改为 Link。P2-1: formatLongDate 默认 locale 改为 zh-CN，weekday 改为 short；返回按钮文案中文化；course-plan-detail 全量中文化；grades/analytics 标题中文化。验证：tsc 0 错误，lint 0 错误，架构图 004/005 已同步。

src/app/(dashboard)/teacher/textbooks/[id]/page.tsx

@@ -31,9 +31,10 @@ export default async function TextbookDetailPage({
     <div className="flex flex-col h-[calc(100vh-4rem)] overflow-hidden">
       {/* Header / Nav (Fixed height) */}
       <div className="flex items-center gap-4 py-4 border-b shrink-0 bg-background z-10">
-        <Button variant="ghost" size="icon" asChild>
-          <Link href="/teacher/textbooks" aria-label="Back to textbooks">
-            <ArrowLeft className="h-4 w-4" aria-hidden="true" />
+        <Button asChild variant="ghost" size="sm">
+          <Link href="/teacher/textbooks">
+            <ArrowLeft className="mr-2 h-4 w-4" aria-hidden="true" />
+            返回教材列表
           </Link>
         </Button>
         <div className="flex-1 min-w-0">