feat(workspace): add workbench layout and traffic totals

Rework the default /workspace layout into a three-column view
with a left-side Workbench, centered terminal, and right-side
status monitor.

Add a new Workbench pane that groups file manager, command
history, and editor into tabs while preserving panel state.
Extend server status data to expose cumulative network upload
and download totals since boot, and show them in the monitor.

Include a lightweight migration for the old default layout and
update related locale strings, pane metadata, and knowledge
base records.

.helloagents/modules/backend.md

@@ -39,6 +39,11 @@
 **行为**: 按 `controller/service/repository/routes` 的分层模式组织连接、通知、设置、快速命令、主题等功能。  
 **结果**: 新增后端能力时应优先延续现有业务域目录结构，而不是在入口文件堆叠逻辑。
 
+### 状态监控
+**条件**: 前端工作区通过 WebSocket 订阅服务器状态。  
+**行为**: `StatusMonitorService` 通过 SSH 读取 `free`、`df`、`/proc/stat` 与 `/proc/net/dev`，同时计算瞬时网速与默认网卡自开机以来的累计上下行字节数。  
+**结果**: 前端状态监控既能展示实时速率，也能展示“开机累计流量”，后续扩展监控字段时应优先复用现有 SSH 采集链路。
+
 ## 依赖关系
 
 ```yaml