chore(workspace): bootstrap platform, judge and frontend

frontend/src/views/HomeView.vue

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+<script setup lang="ts">
+import { computed } from 'vue'
+import { ElButton, ElCard, ElTag } from 'element-plus'
+import { usePlatformHealth } from '../composables/usePlatformHealth'
+
+const { state, version, refresh } = usePlatformHealth()
+const statusText = computed(
+  () =>
+    ({
+      loading: '正在连接平台',
+      connected: '平台已连接',
+      unavailable: '平台暂未连接',
+    })[state.value],
+)
+</script>
+
+<template>
+  <section class="welcome">
+    <el-tag effect="plain"> 开发预览 </el-tag>
+    <h1>从一道题开始，<br />积累每一次进步。</h1>
+    <p class="welcome-description">
+      KF-OnlineJudge 正在建设中。题目练习、提交评测和个人学习记录将在后续版本开放。
+    </p>
+    <el-card shadow="never" class="connection-card">
+      <div class="connection-row">
+        <div>
+          <h2>平台连接</h2>
+          <p aria-live="polite">
+            {{ statusText }}<span v-if="version && state === 'connected'"> · {{ version }}</span>
+          </p>
+        </div>
+        <el-button :loading="state === 'loading'" @click="refresh"> 重新检查 </el-button>
+      </div>
+    </el-card>
+  </section>
+</template>

frontend/src/views/NotFoundView.vue

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+<script setup lang="ts">
+import { ElButton, ElResult } from 'element-plus'
+</script>
+
+<template>
+  <el-result icon="info" title="页面不存在" sub-title="请返回首页继续浏览。">
+    <template #extra>
+      <RouterLink to="/">
+        <el-button type="primary"> 返回首页 </el-button>
+      </RouterLink>
+    </template>
+  </el-result>
+</template>