fix: ExecTerminal 默认 shell 改为 bash（从前端一致）

backend/internal/api/handlers/kubernetes.go

@@ -942,7 +942,7 @@ func (h *KubernetesHandler) ExecTerminal(c *gin.Context) {
 	container := c.Query("container")
 	command := c.Query("command")
 	if command == "" {
-		command = "sh"
+		command = "bash"
 	}
 
 	client, cluster, err := h.getK8sClient(clusterID)