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/routes.ts

@@ -310,13 +310,11 @@ export const admins: RouteRecordRaw = {
       meta: { requiresTeacherAdmin: true },
     },
     {
-      path: "problem/yearly",
-      name: "admin yearly overview",
-      component: () => import("admin/problem/Yearly.vue"),
+      path: "problem/trend",
+      name: "admin grade trend",
+      component: () => import("admin/problem/Trend.vue"),
       meta: { requiresTeacherAdmin: true },
     },
-    // 「年度趋势」改名「历年概况」之前的地址，收藏夹里可能还有
-    { path: "problem/top_ac_trend", redirect: "/admin/problem/yearly" },
     // 题单管理路由
     {
       path: "problemset/list",