LocalMind 1.0.24

Handle stale chat selection and drop model refresh

Refactors sidebar-clearing logic in `MainWindow` into a `ClearSidebarSelection()` helper and now also clears selection when `SelectedChat` points to a chat no longer present in either pinned or regular chat collections. This prevents lingering highlights when chats are removed or become invalid.

Also removes the model-picker `DropDownOpened` refresh path by deleting the XAML event handler, its code-behind method, and `ChatViewModel.RefreshModels()`, simplifying model selection behavior and avoiding implicit refresh-on-open side effects.