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_dev.go

@@ -0,0 +1,14 @@
+//go:build dev
+
+package web
+
+import (
+	"io/fs"
+	"os"
+)
+
+// Dist 在开发模式下直接读取磁盘上的 web/dist（未构建时为空，页面由 Vite 开发服务器提供），
+// 因此无需先构建前端即可 go run
+func Dist() fs.FS {
+	return os.DirFS("web/dist")
+}