Date/time popups

DateFormatter should return a struct `FormattedDate(Start, Separator?, End?, StartLong, EndLong?)` that can be rendered with Blazor, where the longer formats are displayed using a popup

Or we could just replace DateFormatter with FormattedDate

Also show the time zone

Support all-day events

Start by locating DateFormatter and the Blazor rendering entry points. Clarify whether to introduce FormattedDate or replace DateFormatter, then define the representation for time zones, all-day events, and optional date ranges. Done means the selected design supports popup-based long formats and all listed date/time cases.

Written by the indexing model from the issue text.