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/shared/layout/admin.vue

@@ -85,9 +85,9 @@ const NAV: NavGroup[] = [
         role: "teacher",
       },
       {
-        label: "历年概况",
+        label: "年度趋势",
         icon: "ph:chart-line-up",
-        path: "/admin/problem/yearly",
+        path: "/admin/problem/trend",
         role: "teacher",
       },
       { label: "自学情况", icon: "ph:student", path: "/admin/learn", role: "teacher" },