`set_active_window_cursor_style` only matched `WINDOW_CLASS` (used by
`WindowKind::Normal`). Panel-backed windows
(`Floating`/`Dialog`/`PopUp`) use the sibling `PANEL_CLASS`, so the
check failed for them: a focused panel fell through to `mainWindow`,
applying the cursor to the wrong window and leaving `cursor_pointer()`
with no visible effect.

Add an `is_gpui_window` helper that accepts both classes (both carry
`WINDOW_STATE_IVAR`) and use it to resolve the active window.

### Before


https://github.com/user-attachments/assets/f1bf820e-7af8-44f2-bfcd-d51e60706454

### After


https://github.com/user-attachments/assets/d4fa319d-029d-4516-922e-70ed753fa90a

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [ ] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable