feat(exam-homework): add audit report, i18n, error boundaries, and permission hardening

- Add comprehensive audit report for exam and homework module

- Create exam-homework i18n message files (zh-CN + en) and register namespace

- Add permission check to gradeHomeworkSubmissionAction to prevent horizontal privilege escalation

- Add Error Boundary + loading.tsx for 5 key pages (exam build/proctoring, homework assignment/submissions, student assignment)

- Refactor exam-columns to createExamColumns(t) factory for i18n support

- Refactor exam-data-table to manage columns internally via useTranslations

- Replace hardcoded strings with i18n keys in all exam/homework components and pages

- Add getHomeworkSubmissionForGrading data-access for secure grading flow

src/app/(dashboard)/teacher/homework/assignments/create/loading.tsx

@@ -0,0 +1,15 @@
+import { Skeleton } from "@/shared/components/ui/skeleton"
+
+export default function Loading() {
+  return (
+    <div className="flex w-full justify-center items-center min-h-[calc(100vh-160px)] p-8 max-w-[1200px] mx-auto">
+      <div className="w-full space-y-6">
+        <div className="space-y-2">
+          <Skeleton className="h-8 w-48" />
+          <Skeleton className="h-4 w-72" />
+        </div>
+        <Skeleton className="h-[600px] w-full" />
+      </div>
+    </div>
+  )
+}