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/utils/types.ts

@@ -330,8 +330,7 @@ export type {
   StuckProblem,
   StuckProblemDetail,
   StuckStudent,
-  YearlyOverview,
-  YearlySummary,
+  GradeTrendProblem,
   LearnBoard,
   LearnBoardStudent,
   LearnExerciseProgress,