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/messages/en.json

@@ -489,5 +489,24 @@
     "created": "Grow started.",
     "stageAdvanced": "Stage advanced to {stage}.",
     "harvestComplete": "Harvest completed — {grams}g linked to batch."
+  },
+  "notifications": {
+    "title": "Notifications",
+    "markAllRead": "Mark all as read",
+    "noNotifications": "No notifications",
+    "unread": "{count} unread",
+    "types": {
+      "QUOTA_WARNING": "Quota Warning",
+      "BATCH_RECALLED": "Batch Recalled",
+      "DISTRIBUTION_RECORDED": "Distribution Recorded",
+      "SUBSCRIPTION_EXPIRING": "Subscription Expiring"
+    }
+  },
+  "pwa": {
+    "install": "Install as App",
+    "installDesc": "Add CannaManage to your home screen for faster access.",
+    "offline": "No Internet Connection",
+    "offlineDesc": "The app has limited functionality in offline mode.",
+    "retry": "Reconnect"
   }
-}
+}