Use consistent date format

[connium]

Describe the Problem

I would prefer all dates with leading zeros:

Originally posted by @Docmale in #2659 (comment)

This will make the UI more consistent.

Describe Your Preferred Solution

Write all dates in the format dd.mm.yyyy, when using German language.

Describe Possible Alternatives

Keep it as it is.

Related Issues

Additional Information

[andrew8er]

Since we are using a wild mix of date-fns and a half baked temporal polyfill (in PlainDate.ts), I'd like to replace both with a proper modern polyfill like https://github.com/fullcalendar/temporal-polyfill.

I use it in another project, and so far it works great. Also it is easy to replace with the native implementation, once it becomes generally available.