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/src/components/offline-banner.tsx

@@ -0,0 +1,46 @@
+"use client"
+
+import { useEffect, useState } from "react"
+import { onlineManager } from "@tanstack/react-query"
+import { useTranslations } from "next-intl"
+import { WifiOff } from "lucide-react"
+
+import type { ReactNode } from "react"
+
+export function OfflineBanner() {
+  const t = useTranslations("api")
+  const [isOnline, setIsOnline] = useState(true)
+
+  useEffect(() => {
+    // Subscribe to online manager state changes
+    const unsubscribe = onlineManager.subscribe((online) => {
+      setIsOnline(online)
+    })
+    // Set initial state
+    setIsOnline(onlineManager.isOnline())
+    return () => unsubscribe()
+  }, [])
+
+  if (isOnline) return null
+
+  return (
+    <div
+      role="alert"
+      aria-live="polite"
+      className="bg-destructive/10 border-destructive/30 text-destructive flex items-center gap-2 border-b px-4 py-2 text-sm"
+    >
+      <WifiOff className="h-4 w-4 shrink-0" />
+      <span>{t("offline")}</span>
+    </div>
+  )
+}
+
+/**
+ * Wrapper that prevents hydration mismatch for online/offline state.
+ */
+export function OfflineBannerWrapper({ children }: { children: ReactNode }) {
+  const [mounted, setMounted] = useState(false)
+  useEffect(() => setMounted(true), [])
+  if (!mounted) return null
+  return <>{children}</>
+}