fix: 注册MIME类型解决Alpine镜像js文件返回octet-stream的问题

- mime.TypeByExtension在Alpine精简环境中缺少.js等MIME映射
- 使用mime.AddExtensionType在启动时注册常见Web文件类型

backend/cmd/main.go

@@ -225,6 +225,15 @@ func startServer() {
 	r := gin.Default()
 	r.MaxMultipartMemory = 32 << 20
 
+	mime.AddExtensionType(".js", "application/javascript")
+	mime.AddExtensionType(".mjs", "application/javascript")
+	mime.AddExtensionType(".css", "text/css")
+	mime.AddExtensionType(".html", "text/html")
+	mime.AddExtensionType(".json", "application/json")
+	mime.AddExtensionType(".svg", "image/svg+xml")
+	mime.AddExtensionType(".woff", "font/woff")
+	mime.AddExtensionType(".woff2", "font/woff2")
+
 	r.Use(middleware.Logger())
 	r.Use(middleware.Recovery())
 	r.Use(middleware.Cors())