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 |

Start from the Session Spend pane on the /chat path and trace how parent and child sessions currently report cost. Define how cumulative child-session spend should appear by default and whether switching models requires a per-model breakdown. Done means the session view behavior and presentation are agreed for both nested sessions and model changes.

Written by the indexing model from the issue text.