Update "Session Spend" to show more info

Update "Session Spend" to show more info

We should show a cumulative session spend that aggregates all the nested sessions. Like I have parent session that kicks-off child sessions, on the parent session I want to see the aggregated cost which includes those child sessions.

Additionally, if we switch models in a session, can we / should we show spend breakdown per model in that session view?

Maybe some of this can live in an advanced view elsewhere, but the aggregate spend across child sessions should be visible by default in that pane.

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| Field | Value |
| --- | --- |
| App version | 1.0.19 |
| OS | Windows 10.0.26200 |
| Theme | GitHub |
| Path | /chat |
| Tenure | Week 5 |

从 /chat 路径上的 Session Spend 面板开始，追踪父会话和子会话当前如何报告成本。定义默认情况下应如何显示子会话的累计支出，以及切换模型是否需要按模型细分。对于嵌套会话和模型变更，若会话视图的行为和呈现方式都已达成一致，则视为完成。

由索引模型根据 Issue 内容生成。