feat(frontend): 搭建桌面端基础界面与 Workspace

- App Shell 壳层：主导航、次导航、Title Bar、状态栏
- Vault 入口页：最近 Vault、打开/创建 Vault、AI Core 状态
- Workspace 与文件树：浏览、打开、创建笔记/文件夹
- Design Token：浅色/深色主题 CSS Variables
- Contracts / Service / Store / Router 基础架构
- 统一 ApiClient 与 SseClient，对接后端 API 契约

frontend/src/components/common/AppShell.vue

@@ -0,0 +1,87 @@
+<script setup lang="ts">
+import { computed } from 'vue'
+import { useRoute, useRouter } from 'vue-router'
+import { useWorkspaceStore } from '@/stores/workspace'
+import { useThemeStore } from '@/stores/theme'
+import { useEditorStore } from '@/stores/editor'
+import { useSettingsStore } from '@/stores/settings'
+import { useAgentStore } from '@/stores/agent'
+import PrimarySidebar from './PrimarySidebar.vue'
+import SecondarySidebar from './SecondarySidebar.vue'
+import StatusBar from './StatusBar.vue'
+import TitleBar from './TitleBar.vue'
+
+defineProps<{
+  showSecondarySidebar?: boolean
+}>()
+
+const workspaceStore = useWorkspaceStore()
+const themeStore = useThemeStore()
+const editorStore = useEditorStore()
+const settingsStore = useSettingsStore()
+const agentStore = useAgentStore()
+const route = useRoute()
+const router = useRouter()
+
+const routeName = computed(() => route.name as string)
+
+const secondaryComponent = computed(() => {
+  switch (routeName) {
+    case 'workspace': return 'file-tree'
+    case 'search': return 'search-filters'
+    case 'chat': return 'conversation-list'
+    case 'agent': return 'run-list'
+    case 'tasks': return 'task-filters'
+    case 'skills':
+    case 'plugins': return 'extension-list'
+    default: return null
+  }
+})
+
+function openCitation(noteId: string, blockId: string, filePath: string) {
+  workspaceStore.openFile(filePath)
+  editorStore.highlightBlock(blockId)
+  router.push('/workspace')
+}
+
+defineExpose({ openCitation })
+</script>
+
+<template>
+  <div class="app-shell" :class="{ 'theme-dark': themeStore.isDark }">
+    <TitleBar />
+    <div class="app-body">
+      <PrimarySidebar />
+      <SecondarySidebar v-if="secondaryComponent" :component="secondaryComponent" />
+      <main class="main-content">
+        <slot />
+      </main>
+    </div>
+    <StatusBar />
+  </div>
+</template>
+
+<style scoped>
+.app-shell {
+  display: flex;
+  flex-direction: column;
+  height: 100vh;
+  width: 100vw;
+  background: var(--color-background-primary);
+  color: var(--color-text-primary);
+}
+
+.app-body {
+  flex: 1;
+  display: flex;
+  min-height: 0;
+  overflow: hidden;
+}
+
+.main-content {
+  flex: 1;
+  min-width: 0;
+  overflow: hidden;
+  background: var(--color-background-primary);
+}
+</style>