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/package-lock.json

@@ -11,7 +11,8 @@
         "lucide-react": "^0.468.0",
         "react": "^18.3.1",
         "react-dom": "^18.3.1",
-        "react-router-dom": "^6.28.0"
+        "react-router-dom": "^6.28.0",
+        "rrule": "^2.8.1"
       },
       "devDependencies": {
         "@types/react": "^18.3.1",
@@ -5047,6 +5048,15 @@
         "fsevents": "~2.3.2"
       }
     },
+    "node_modules/rrule": {
+      "version": "2.8.1",
+      "resolved": "https://registry.npmjs.org/rrule/-/rrule-2.8.1.tgz",
+      "integrity": "sha512-hM3dHSBMeaJ0Ktp7W38BJZ7O1zOgaFEsn41PDk+yHoEtfLV+PoJt9E9xAlZiWgf/iqEqionN0ebHFZIDAp+iGw==",
+      "license": "BSD-3-Clause",
+      "dependencies": {
+        "tslib": "^2.4.0"
+      }
+    },
     "node_modules/safe-array-concat": {
       "version": "1.1.4",
       "resolved": "https://registry.npmjs.org/safe-array-concat/-/safe-array-concat-1.1.4.tgz",
@@ -5548,6 +5558,12 @@
         "url": "https://github.com/sponsors/SuperchupuDev"
       }
     },
+    "node_modules/tslib": {
+      "version": "2.8.1",
+      "resolved": "https://registry.npmjs.org/tslib/-/tslib-2.8.1.tgz",
+      "integrity": "sha512-oJFu94HQb+KVduSUQL7wnpmqnfmLsOA/nAh6b6EH0wCEoK0/mPeXU6c3wKDV83MkOuHPRHtSXKKU99IBazS/2w==",
+      "license": "0BSD"
+    },
     "node_modules/type-fest": {
       "version": "0.16.0",
       "resolved": "https://registry.npmjs.org/type-fest/-/type-fest-0.16.0.tgz",