引入二级大纲功能，新增父子节点关系及子节点排序，同时调整API与UI以支持嵌套大纲管理

frontend/src/views/NovelWorkspace.vue

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+<script setup lang="ts">
+import { ref, onMounted, provide } from 'vue'
+import { useRoute, useRouter } from 'vue-router'
+import { fetchNovel } from '@/api/novels'
+import { useToast } from '@/composables/useToast'
+import type { Novel } from '@/types/novel'
+import NovelSubNav from '@/components/novel/NovelSubNav.vue'
+
+const route = useRoute()
+const router = useRouter()
+const toast = useToast()
+
+const novelId = Number(route.params.id)
+const novel = ref<Novel | null>(null)
+const loading = ref(true)
+
+/** 子页面可设置的副标题（如 "共 5 个节点"） */
+const subtitle = ref('')
+
+async function load() {
+  loading.value = true
+  try {
+    novel.value = await fetchNovel(novelId)
+  } catch {
+    toast.error('作品不存在')
+    router.push({ name: 'home' })
+  } finally {
+    loading.value = false
+  }
+}
+
+// 提供给子路由
+provide('workspace-novel', novel)
+provide('workspace-novel-id', novelId)
+provide('workspace-subtitle', subtitle)
+
+onMounted(load)
+</script>
+
+<template>
+  <div v-if="!loading && novel" class="workspace">
+    <NovelSubNav
+      :novel-id="novelId"
+      :novel-title="novel.title"
+      :subtitle="subtitle"
+    >
+      <template #actions>
+        <!-- 子页面通过 Teleport 注入操作按钮 -->
+        <div id="workspace-actions" />
+      </template>
+    </NovelSubNav>
+
+    <div class="workspace-content">
+      <RouterView />
+    </div>
+  </div>
+
+  <div v-else-if="loading" class="workspace-loading">
+    <div class="loading-pulse" style="height: 1.5rem; width: 30%; margin-bottom: var(--space-md)" />
+    <div class="loading-pulse" style="height: 1rem; width: 60%; margin-bottom: var(--space-2xl)" />
+    <div class="loading-pulse" style="height: 20rem; width: 100%" />
+  </div>
+</template>
+
+<style scoped>
+.workspace {
+  max-width: 900px;
+  margin: 0 auto;
+}
+
+.workspace-content {
+  animation: fadeInUp var(--duration-normal) var(--ease-out-smooth) both;
+}
+
+.workspace-loading {
+  max-width: 900px;
+  margin: var(--space-3xl) auto;
+}
+
+.loading-pulse {
+  background: linear-gradient(
+    90deg,
+    var(--paper-100) 25%,
+    var(--paper-200) 50%,
+    var(--paper-100) 75%
+  );
+  background-size: 200% 100%;
+  border-radius: var(--radius-sm);
+  animation: shimmer 1.5s ease infinite;
+}
+</style>