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/package.json

@@ -36,6 +36,7 @@
     "@radix-ui/react-slot": "1.1.1",
     "@radix-ui/react-toast": "1.2.1",
     "@radix-ui/react-tooltip": "1.1.5",
+    "@tanstack/react-query": "^5.101.0",
     "@tanstack/react-table": "^8.21.3",
     "class-variance-authority": "0.7.1",
     "clsx": "2.1.1",