feat: add teaching enhancement features

1. AI personalized hints after 3 failures (streaming SSE)
2. Submission error distribution panel in "my submissions" tab
3. Similar problem recommendations on AC or 3+ failures
4. Admin stuck problems analysis page

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/admin/api.ts

@@ -434,3 +434,8 @@ export function getProblemSetProgress(problemSetId: number) {
 export function removeUserFromProblemSet(problemSetId: number, userId: number) {
   return http.delete(`admin/problemset/${problemSetId}/progress/${userId}`)
 }
+
+// 学生卡点分析
+export function getStuckProblems() {
+  return http.get("admin/problem/stuck")
+}