fix(setup): 支持从配置文件读取 Setup Wizard 监听地址

Setup Wizard 之前硬编码使用 8080 端口，现在支持从 config.yaml 或
环境变量 (SERVER_HOST, SERVER_PORT) 读取监听地址，方便用户在端口
被占用时使用其他地址启动初始化向导。

backend/cmd/server/main.go

@@ -15,6 +15,7 @@ import (
 	"syscall"
 	"time"
 
+	"sub2api/internal/config"
 	"sub2api/internal/handler"
 	"sub2api/internal/middleware"
 	"sub2api/internal/setup"
@@ -94,8 +95,10 @@ func runSetupServer() {
 		r.Use(web.ServeEmbeddedFrontend())
 	}
 
-	addr := ":8080"
-	log.Printf("Setup wizard available at http://localhost%s", addr)
+	// Get server address from config.yaml or environment variables (SERVER_HOST, SERVER_PORT)
+	// This allows users to run setup on a different address if needed
+	addr := config.GetServerAddress()
+	log.Printf("Setup wizard available at http://%s", addr)
 	log.Println("Complete the setup wizard to configure Sub2API")
 
 	if err := r.Run(addr); err != nil {