build: support running the server with go run during development

- The dev build tag reads web/dist from disk instead of embedding it,
  so the backend starts without building the frontend first; Vite
  serves the UI and proxies /api and WebSocket to the Go server
- Document the two-terminal dev workflow and ignore .dev-data/

web/embed.go

@@ -1,4 +1,7 @@
+//go:build !dev
+
 // Package web 把前端构建产物（web/dist，由 npm run build 生成）嵌入 Go 二进制。
+// 开发时使用 -tags dev，前端由 Vite 开发服务器提供，见 embed_dev.go。
 package web
 
 import (