fix(frontend): 修复合并审阅发现的构建与契约问题

恢复 Vue TypeScript 生产构建，补齐可运行页面壳子，并修复文件树与 SSE 状态问题。

按 FastAPI Wire Contract 统一 Service DTO 映射，同时补充前端开发说明和问题修复复盘。

frontend/src/features/editor/EditorHeader.vue

@@ -0,0 +1,26 @@
+<script setup lang="ts">
+import { useEditorStore } from '@/stores/editor'
+import { useWorkspaceStore } from '@/stores/workspace'
+
+const editorStore = useEditorStore()
+const workspaceStore = useWorkspaceStore()
+</script>
+
+<template>
+  <header class="editor-header">
+    <strong>{{ workspaceStore.activeFile?.name ?? '未命名笔记' }}</strong>
+    <span>{{ editorStore.saveStatus }}</span>
+  </header>
+</template>
+
+<style scoped>
+.editor-header {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  min-height: 40px;
+  padding: 0 var(--space-lg);
+  border-bottom: 1px solid var(--color-border-subtle);
+  color: var(--color-text-secondary);
+}
+</style>