front: gov: drag and drop the selected train displays the first train of the timetable for 1s

[JulesPeignier]

What happened?

When trying to change the track occupancy of a train in the gov, the gov displays the first train in the schedule for one second:

20260806-1304-12.3338712.mp4

What did you expect to happen?

I expect that a train I haven't selected won't appear

How can we reproduce it (as minimally and precisely as possible)?

1. create a scenario with 2 trains
2. modify the track occupancy (drag and drop) of the latest train in the gov of a station

On which environments the bug occurs?

Recette (SNCF)

On which browser the bug occurs?

Firefox

OSRD version (top right corner Account button > Informations)

staging 87d2bdd