Recurring event editing (rrule) with this/all occurrence scope, web push
subscriptions (VAPID) with a cached config layer for notification settings,
and email delivery via nodemailer.
Also fixes the calendar view never actually stacking on mobile: the
Calendars filter column used flex:1 with minWidth:0 on its sibling, so
flex-wrap never triggered regardless of viewport width, squeezing the grid
and view switcher into a sliver next to a fixed 220px sidebar. Adds a
proper mobile breakpoint that stacks the layout, scrolls the week grid
horizontally instead of compressing it, and enlarges touch targets.
