Print configurator: Preselect period in single-period camps

[carlobeltrame]

Also sets the period selection dropdown to readonly, with appropriate style (hide the caret).

This has a small risk of some users still having print configs in their local storage with some empty period selection. However, our prints can handle this, and the users can easily fix their print config themselves if needed. Fixed

Todo:

• When auto-repairing a config, make sure to also select the sole period

[github-actions]

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[BacLuc]

Minor inconvenience:
If you delete the period that was saved in the print config, the print configurator does not like it:

But if you reset it, it works again.

[carlobeltrame]

Minor inconvenience: If you delete the period that was saved in the print config, the print configurator does not lake it

Should be fixed in a090765