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/src/__tests__/components/offline-banner.test.tsx

@@ -1,6 +1,6 @@
+import React from "react"
 import { render, screen } from "@testing-library/react"
 import { describe, expect, it, vi } from "vitest"
-import React from "react"
 
 // Since there's no existing offline banner component, we'll create and test a minimal one
 // This tests the pattern that would be used for an offline banner