feat(sprint-5): Phase 2 — React Query API client layer

- @tanstack/react-query with QueryClientProvider in providers/index.tsx
- Typed api-client.ts fetch wrapper with ApiError class + apiDownload
- Service modules: members, distributions, stock, reports, dashboard, portal, staff
- Offline banner component (onlineManager subscription)
- API error boundary with retry button
- Loading skeleton components (card, table, chart, form, dashboard)
- i18n for error/loading states (de/en)

cannamanage-frontend/messages/en.json

@@ -292,5 +292,17 @@
     "allMonths": "All months",
     "footerText": "Cannabis cultivation club — Secure member management",
     "adminLogin": "Go to Admin Login"
+  },
+  "api": {
+    "loading": "Loading...",
+    "error": "Failed to load data.",
+    "retry": "Try again",
+    "offline": "No connection to server — data may be outdated.",
+    "networkError": "Network error. Please check your connection.",
+    "unauthorized": "Session expired. Please sign in again.",
+    "forbidden": "You do not have permission for this action.",
+    "notFound": "Resource not found.",
+    "quotaExceeded": "Quota exceeded.",
+    "serverError": "Server error. Please try again later."
   }
 }