feat(sprint-6): Phase 6 — Notifications (WebSocket) + PWA

- WebSocket: Spring STOMP + SockJS, NotificationService, persistent notifications table
- NotificationController: GET/PUT endpoints for notification management
- Frontend: notification bell with unread badge, dropdown panel, real-time via STOMP
- PWA: manifest.json, service worker (manual sw.js), offline page, install prompt
- PWA icons (192+512), dark theme colors, standalone display
- Full i18n (de/en) for notifications and PWA
- Flyway V10 migration for notifications table
- spring-boot-starter-websocket dependency added

cannamanage-frontend/e2e/mock-backend.mjs

@@ -16,7 +16,10 @@ const MOCK_USER = {
 const server = http.createServer((req, res) => {
   // CORS headers
   res.setHeader("Access-Control-Allow-Origin", "*")
-  res.setHeader("Access-Control-Allow-Methods", "GET, POST, PUT, DELETE, OPTIONS")
+  res.setHeader(
+    "Access-Control-Allow-Methods",
+    "GET, POST, PUT, DELETE, OPTIONS"
+  )
   res.setHeader("Access-Control-Allow-Headers", "Content-Type, Authorization")
 
   if (req.method === "OPTIONS") {