feat(dashboard/offline): persist React Query cache for offline reads

First slice of offline-first (Phase 1). Makes every dashboard area *viewable*
offline with last-synced data, instead of empty lists on an offline reload
(previously only next-pwa's 10-min API cache survived).

- offline-db: add a generic `kv` IndexedDB store (DB v2, preserves order_queue)
  with kvGet/kvSet/kvDelete; all degrade silently on quota/unavailable.
- query-persister: debounced snapshot of the React Query cache via
  dehydrate/hydrate (no new dependency). Restore is guarded by a schema buster,
  24h max-age, and a café scope so one tenant never hydrates another's data.
- providers: gcTime 24h so hydrated data isn't GC'd; restore on mount + persist
  on cache changes, re-scoped when the signed-in café changes.

No write-path changes; the existing POS order queue is untouched. Next in
Phase 1: generalize that queue into an idempotent outbox with client→server
ID remapping.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/dashboard/src/components/providers.tsx

@@ -1,20 +1,46 @@
 "use client";
 
 import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
-import { useState } from "react";
+import { useEffect, useState } from "react";
 import { ConfirmProvider } from "@/components/providers/confirm-provider";
 import { MeeziToaster } from "@/components/ui/meezi-toaster";
+import { useAuthStore } from "@/lib/stores/auth.store";
+import { restoreQueryCache, startPersisting } from "@/lib/offline/query-persister";
 
 export function Providers({ children }: { children: React.ReactNode }) {
   const [queryClient] = useState(
     () =>
       new QueryClient({
         defaultOptions: {
-          queries: { staleTime: 30_000, retry: 1 },
+          queries: {
+            staleTime: 30_000,
+            retry: 1,
+            // Keep data in memory long enough to back offline reads; it is also
+            // persisted to IndexedDB by the persister below.
+            gcTime: 24 * 60 * 60 * 1000,
+          },
         },
       })
   );
 
+  // Persist the query cache to IndexedDB so the dashboard is viewable offline.
+  // Scoped to the current café so a different tenant never hydrates this data.
+  const cafeId = useAuthStore((s) => s.user?.cafeId);
+  useEffect(() => {
+    const scope = cafeId ?? "anon";
+    let active = true;
+    let stop: () => void = () => {};
+    void (async () => {
+      await restoreQueryCache(queryClient, scope);
+      if (!active) return;
+      stop = startPersisting(queryClient, scope);
+    })();
+    return () => {
+      active = false;
+      stop();
+    };
+  }, [queryClient, cafeId]);
+
   return (
     <QueryClientProvider client={queryClient}>
       <ConfirmProvider>