[Bug] Switching accounts does not refresh session history

When using multiple accounts, switching from one account to another does not refresh the session/chat history. After changing accounts, the application continues to display conversations from the previously selected account instead of loading the sessions associated with the newly selected account.

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| Field | Value |
| --- | --- |
| App version | 1.0.6 |
| OS | Windows 10.0.26200 |
| Theme | Notionish |
| Path | / |
| Tenure | Day 2 (Week 1) |

Start by reproducing the issue in the desktop app at the root path with two accounts: switch accounts and observe whether the displayed session history changes. Trace the account-switch and session-history loading entry points, then verify that only conversations belonging to the newly selected account are shown.

索引モデルが issue の本文から書いたものです。