feat(app): scaffold Vue3 frontend (stage 1)

app-frontend/src/pages/RegisterPage.vue

@@ -0,0 +1,68 @@
+<script setup>
+import { useRouter } from 'vue-router'
+
+const router = useRouter()
+
+function goLogin() {
+  router.push('/login')
+}
+</script>
+
+<template>
+  <div class="auth-wrap">
+    <el-card shadow="never" class="auth-card" :body-style="{ padding: '22px' }">
+      <div class="brand">
+        <div class="brand-title">知识管理平台</div>
+        <div class="brand-sub app-muted">用户注册</div>
+      </div>
+
+      <el-alert
+        type="info"
+        :closable="false"
+        title="阶段1：仅完成前台工程与布局搭建。注册/邮箱验证等功能将在后续阶段迁移。"
+        show-icon
+      />
+
+      <div class="actions">
+        <el-button @click="goLogin">返回登录</el-button>
+      </div>
+    </el-card>
+  </div>
+</template>
+
+<style scoped>
+.auth-wrap {
+  min-height: 100vh;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  padding: 20px;
+}
+
+.auth-card {
+  width: 100%;
+  max-width: 420px;
+  border-radius: var(--app-radius);
+  border: 1px solid var(--app-border);
+  box-shadow: var(--app-shadow);
+}
+
+.brand {
+  margin-bottom: 14px;
+}
+
+.brand-title {
+  font-size: 18px;
+  font-weight: 900;
+}
+
+.brand-sub {
+  margin-top: 4px;
+  font-size: 12px;
+}
+
+.actions {
+  margin-top: 16px;
+}
+</style>
+