Original PR description

Infinite re-rendering in floor_screen.

Root cause: `getFirstAppointment` mutates reactive model state
(appointment.start) during rendering:

```
appointments.map((appointment) => {
    if (appointment.start < startOfToday) {
        appointment.start = startOfToday;  // <= mutates reactive state!
    }
});
```

And `startOfToday` is set by
`DateTime.now().set({ hours: 0, minutes: 0, seconds: 0 })`

Which doesn't zero milliseconds, so each render creates a new
`startOfToday` with a later millisecond value.

The comparison `appointment.start < startOfToday` keeps being true
triggers another write => another re-render => infinite loop.