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/services/index.ts

@@ -1,5 +1,5 @@
 export { apiClient, ApiErrorClass } from './apiClient'
-export type { ApiError } from './apiClient'
+export type { ApiError } from '@/contracts'
 export { SseClient } from './sseClient'
 export type { SseClientOptions, SseEventHandler } from './sseClient'
 export * as noteService from './noteService'