style(header): 演示模式菜单文案改为「进入演示 / 退出演示」

两端都是四字，动作对称。spec 与 plan 文档同步更新。

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/shared/components/Beian.vue

@@ -29,7 +29,6 @@ const hiddenICP = computed(() =>
   ["problem", "contest problem"].includes(route.name as string),
 )
 
-
 const currentYear = new Date().getFullYear()
 const copyrightText = `© 2022 - ${currentYear} 判题狗 保留所有权利`
 
@@ -56,7 +55,6 @@ function goPublicSecurity() {
   margin: 12px 0;
 }
 
-
 .beian--mobile {
   font-size: 12px;
   max-width: 100%;

src/shared/components/Header.vue

@@ -227,7 +227,7 @@ const options = computed<Array<DropdownOption | DropdownDividerOption>>(() => [
     },
   },
   {
-    label: userStore.demoMode ? "退出演示" : "学生视角",
+    label: userStore.demoMode ? "退出演示" : "进入演示",
     key: "demo-mode",
     show: userStore.canToggleDemoMode,
     icon: renderIcon("fluent-emoji:graduation-cap"),