Update Swift options usage per framework interface changes.

Yet again, time to play optionals whack-a-mole.
Fix end date not updating for tomorrow events.

This is because of a defect in bad end date detection.
Fix updating of Months screen section headers for new events.

Fixup event save notification handling. Refactor.
Temporarily disable date picking for existing events.

Supporting it would require updating the
structured events collection and ensuring the UI
can update safely and accordingly. For example,
the Day screen must no longer show the event, but
this would break existing transitions.
Fetch events whenever app becomes active on Months screen.

Refactor.
Minor cleanup.
Prevent crashing when dismissing to Day screen.

This happens when date gets edited on Events screen.

Attempt to limit constant overwriting of data source.