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/

.gitignore

@@ -5,3 +5,6 @@ docs/
 # 前端
 node_modules/
 web/dist/
+
+# 本地开发数据
+.dev-data/