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/public/manifest.json

@@ -0,0 +1,24 @@
+{
+  "name": "CannaManage",
+  "short_name": "CannaManage",
+  "description": "Cannabis-Club Verwaltung",
+  "start_url": "/",
+  "display": "standalone",
+  "background_color": "#0D1117",
+  "theme_color": "#2ECC71",
+  "orientation": "portrait-primary",
+  "icons": [
+    {
+      "src": "/icons/icon-192.png",
+      "sizes": "192x192",
+      "type": "image/png",
+      "purpose": "any maskable"
+    },
+    {
+      "src": "/icons/icon-512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "any maskable"
+    }
+  ]
+}

cannamanage-frontend/public/sw.js

@@ -0,0 +1,72 @@
+/// <reference lib="webworker" />
+
+const CACHE_NAME = "cannamanage-v1"
+const OFFLINE_URL = "/offline"
+
+// Assets to pre-cache
+const PRECACHE_ASSETS = [
+  "/offline",
+  "/manifest.json",
+  "/icons/icon-192.png",
+  "/icons/icon-512.png",
+]
+
+self.addEventListener("install", (event) => {
+  event.waitUntil(
+    caches.open(CACHE_NAME).then((cache) => {
+      return cache.addAll(PRECACHE_ASSETS)
+    })
+  )
+  self.skipWaiting()
+})
+
+self.addEventListener("activate", (event) => {
+  event.waitUntil(
+    caches.keys().then((cacheNames) => {
+      return Promise.all(
+        cacheNames
+          .filter((name) => name !== CACHE_NAME)
+          .map((name) => caches.delete(name))
+      )
+    })
+  )
+  self.clients.claim()
+})
+
+self.addEventListener("fetch", (event) => {
+  // Only handle GET requests
+  if (event.request.method !== "GET") return
+
+  // Skip API requests — let them fail naturally
+  if (event.request.url.includes("/api/")) return
+
+  // Network-first for navigation requests
+  if (event.request.mode === "navigate") {
+    event.respondWith(
+      fetch(event.request).catch(() => {
+        return caches.match(OFFLINE_URL)
+      })
+    )
+    return
+  }
+
+  // Stale-while-revalidate for static assets
+  if (
+    event.request.destination === "style" ||
+    event.request.destination === "script" ||
+    event.request.destination === "image"
+  ) {
+    event.respondWith(
+      caches.match(event.request).then((cachedResponse) => {
+        const fetchPromise = fetch(event.request).then((networkResponse) => {
+          if (networkResponse && networkResponse.status === 200) {
+            const cache = caches.open(CACHE_NAME)
+            cache.then((c) => c.put(event.request, networkResponse.clone()))
+          }
+          return networkResponse
+        })
+        return cachedResponse || fetchPromise
+      })
+    )
+  }
+})