Add recurring events, push notifications, config sync — and mobile layout fixes

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.

frontend/src/components/views/MonthGrid.tsx

@@ -7,7 +7,7 @@ export default function MonthGrid({ events, date, onSelectDate, onSelectEvent }:
   events: EventSummary[]
   date: Date
   onSelectDate?: (d: Date) => void
-  onSelectEvent: (id: number) => void
+  onSelectEvent: (ev: EventSummary) => void
 }) {
   const days = monthGridDays(date)
   const today = new Date()
@@ -35,7 +35,7 @@ export default function MonthGrid({ events, date, onSelectDate, onSelectEvent }:
                 className="cal-event-chip"
                 style={{ background: ev.calendar_color, color: '#fff' }}
                 title={ev.title}
-                onClick={e => { e.stopPropagation(); onSelectEvent(ev.id) }}
+                onClick={e => { e.stopPropagation(); onSelectEvent(ev) }}
               >
                 {!ev.all_day && `${formatTime(ev.start_at)} `}{ev.title}
               </span>