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/features/workspace/WorkspaceView.vue

@@ -0,0 +1,72 @@
+<script setup lang="ts">
+import { onMounted } from 'vue'
+import { useWorkspaceStore } from '@/stores/workspace'
+import { useEditorStore } from '@/stores/editor'
+import EditorHeader from '@/features/editor/EditorHeader.vue'
+import EditorPane from '@/features/editor/EditorPane.vue'
+
+const workspaceStore = useWorkspaceStore()
+const editorStore = useEditorStore()
+
+onMounted(() => {
+  if (!workspaceStore.fileTree.length && workspaceStore.hasVault) {
+    // Already loaded
+  }
+  if (!workspaceStore.activeFilePath && workspaceStore.fileTree.length === 0) {
+    workspaceStore.openFile('/欢迎使用知笔知己.md')
+    editorStore.loadFile('/欢迎使用知笔知己.md')
+  }
+})
+</script>
+
+<template>
+  <div class="workspace-view">
+    <template v-if="workspaceStore.activeFilePath">
+      <EditorHeader />
+      <EditorPane />
+    </template>
+    <div v-else class="empty-workspace">
+      <div class="empty-content">
+        <div class="empty-icon">📝</div>
+        <h2>开始写作</h2>
+        <p>从左侧文件树选择笔记，或创建新的笔记</p>
+      </div>
+    </div>
+  </div>
+</template>
+
+<style scoped>
+.workspace-view {
+  display: flex;
+  flex-direction: column;
+  height: 100%;
+  overflow: hidden;
+}
+
+.empty-workspace {
+  flex: 1;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  color: var(--color-text-tertiary);
+}
+
+.empty-content {
+  text-align: center;
+
+  h2 {
+    font-size: 18px;
+    color: var(--color-text-secondary);
+    margin: 12px 0 8px;
+  }
+
+  p {
+    font-size: 14px;
+  }
+}
+
+.empty-icon {
+  font-size: 48px;
+  opacity: 0.5;
+}
+</style>