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/services/notifications.ts

@@ -0,0 +1,32 @@
+import { apiClient } from "@/lib/api-client"
+
+export interface Notification {
+  id: string
+  type: string
+  title: string
+  message: string
+  link: string
+  read: boolean
+  createdAt: string
+}
+
+export interface NotificationsResponse {
+  notifications: Notification[]
+  unreadCount: number
+}
+
+export async function getNotifications(): Promise<NotificationsResponse> {
+  return apiClient<NotificationsResponse>("/notifications")
+}
+
+export async function markNotificationAsRead(id: string): Promise<void> {
+  await apiClient<void>(`/notifications/${id}/read`, { method: "PUT" })
+}
+
+export async function markAllNotificationsAsRead(): Promise<{
+  updated: number
+}> {
+  return apiClient<{ updated: number }>("/notifications/read-all", {
+    method: "PUT",
+  })
+}