Support local timezones for DateTime

[modersohn]

This is mainly for me to remember. Once we're offering a CSV transaction export of a given date range, it would be important to convert that from the timezone of the user to UTC before applying it to the filtering (see also #57).

This article has a nice step-by-step on how to achieve this: https://www.meziantou.net/convert-datetime-to-user-s-time-zone-with-server-side-blazor.htm