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/lesson-plans/new/page.tsx

@@ -10,12 +10,13 @@ export default function NewLessonPlanPage(): JSX.Element {
   return (
     <div className="p-6">
       <div className="mb-6 flex items-center gap-4">
-        <Button asChild variant="ghost" size="icon">
-          <Link href="/teacher/lesson-plans" aria-label="Back to lesson plans">
-            <ArrowLeft className="h-4 w-4" aria-hidden="true" />
+        <Button asChild variant="ghost" size="sm">
+          <Link href="/teacher/lesson-plans">
+            <ArrowLeft className="mr-2 h-4 w-4" aria-hidden="true" />
+            返回备课列表
           </Link>
         </Button>
-        <h1 className="text-2xl font-bold tracking-tight">New Lesson Plan</h1>
+        <h1 className="text-2xl font-bold tracking-tight">新建课案</h1>
       </div>
       <TemplatePicker />
     </div>