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.

backend/package.json

@@ -17,6 +17,9 @@
     "ical.js": "^2.1.0",
     "ics": "^3.8.1",
     "jose": "^5.9.6",
-    "pg": "^8.13.1"
+    "nodemailer": "^9.0.3",
+    "pg": "^8.13.1",
+    "rrule": "^2.8.1",
+    "web-push": "^3.6.7"
   }
 }