fix(前台): 地址打错不再是一整页空白，题目、比赛、题单不存在时写明是哪样不存在、给一个回去的按钮

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

apps/web/src/shared/components/NotFound.vue

@@ -0,0 +1,71 @@
+<script setup lang="ts">
+import { Icon } from "@iconify/vue"
+import { useThemeVars } from "naive-ui"
+import type { RouteLocationRaw } from "vue-router"
+
+/**
+ * 「找不到」的整页提示：地址打错、题目 / 比赛 / 题单不存在或没公开、比赛还没开始。
+ * 原来各页各写各的 —— 打错地址是一整页空白、题号不对只有一个「无数据」，学生看不出出了什么事。
+ * 样子跟提交详情的「这条提交不存在」一致：图标、一句话、一句为什么、一个回去的按钮。
+ */
+const props = withDefaults(
+  defineProps<{
+    title: string
+    hint?: string
+    icon?: string
+    to?: RouteLocationRaw
+    toLabel?: string
+  }>(),
+  { hint: "", icon: "ph:magnifying-glass", to: "/", toLabel: "回首页" },
+)
+
+const theme = useThemeVars()
+const router = useRouter()
+</script>
+
+<template>
+  <div class="not-found">
+    <Icon :icon="props.icon" :width="40" class="icon" />
+    <span class="title">{{ props.title }}</span>
+    <span v-if="props.hint" class="hint">{{ props.hint }}</span>
+    <div class="actions">
+      <slot>
+        <n-button type="primary" @click="router.push(props.to)">{{ props.toLabel }}</n-button>
+      </slot>
+    </div>
+  </div>
+</template>
+
+<style scoped>
+.not-found {
+  min-height: 360px;
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  justify-content: center;
+  gap: 10px;
+  padding: 48px 16px;
+  text-align: center;
+}
+
+.icon {
+  color: v-bind("theme.textColor3");
+  opacity: 0.7;
+}
+
+.title {
+  font-size: 18px;
+  font-weight: 600;
+}
+
+.hint {
+  font-size: var(--oj-fs-sec);
+  color: v-bind("theme.textColor3");
+}
+
+.actions {
+  display: flex;
+  gap: 10px;
+  margin-top: 8px;
+}
+</style>