feat(年度趋势): 后台年度趋势改成「这届和往届比」，比往届差的题排在最前面

- 选一届和往届（或另一届）比，两边都至少 20 人做过的题才比；分比往届差 / 差不多 / 比往届好三组，一题一根哑铃条
- 侧栏「历年概况」改回「年度趋势」，地址换成 /admin/problem/trend

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/web/src/admin/api.ts

@@ -9,7 +9,7 @@ import type {
   AdminAiReportList,
   StuckProblem,
   StuckProblemDetail,
-  YearlyOverview,
+  GradeTrendProblem,
   LearnBoard,
   LearnExerciseProgressList,
   LearnExerciseAttempt,
@@ -624,8 +624,8 @@ export function getLearnExerciseAttempts(exerciseId: number, params: { className
   })
 }
 
-export function getYearlyOverview() {
-  return api.get<YearlyOverview>("admin/problem-analytics/yearly")
+export function getGradeTrend() {
+  return api.get<GradeTrendProblem[]>("admin/problem-analytics/grades")
 }
 
 // AI 学习分析报告