Holidays do not allow to add time

Take the 2025-12-26 as holiday and try to add a 2 h interval via

```sh
timew track 2025-12-26T8:00 for 2h work
```

which gives

```out
You cannot overlap intervals. Correct the start/end time, or specify the :adjust hint.
```

Start by reproducing the issue with the `timew track 2025-12-26T8:00 for 2h work` command and a 2025-12-26 holiday. Trace the holiday handling and overlap validation to determine why the interval is rejected. Done means a two-hour interval can be added on that holiday without an incorrect overlap error, while genuine overlaps remain protected.

Written by the indexing model from the issue text.