Support for VALARM

[sylvinus]

RemindersSection.tsx exists with full VALARM UI — but it's not wired into EventModal (no
import found)

+implement notification delivery (push and/or email and/or web notifs).

[PanchoutNathan]

Verify if sabre/DAV can do this

[sylvinus]

VALARM should only be allowed in the calendar owned by the user