feat(dashboard): Next.js 16 merchant panel with offline POS and PWA

Complete merchant dashboard upgrade:

Next.js 16 compatibility:
- Fix params/searchParams typed as Promise<{}> throughout App Router
- Replace middleware.ts with proxy.ts (Next.js 16 convention)
- Remove unused @ts-expect-error directives caught by stricter TS
- Cast dynamic next-intl t() keys to fix TranslateArgs type errors

Offline POS:
- IndexedDB queue (meezi_pos_offline) for orders created while offline
- Zustand sync store tracking queueCount, isSyncing, isOnline
- useOfflineSync hook: auto-syncs on reconnect/visibility-change
- SyncStatusIndicator chip in topbar (amber=offline, blue=syncing)
- submitOrderToApi falls back to local order on network failure
- Local orders skip payment flow; sync on reconnect

PWA (installable):
- @ducanh2912/next-pwa with Workbox runtime caching rules
- Web App Manifest (manifest.ts) — RTL/Farsi, theme #0F6E56
- PWA icons: 192px, 512px, maskable 512px
- next.config.ts replaces next.config.mjs

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

web/dashboard/src/lib/notify.ts

@@ -0,0 +1,56 @@
+import { toast } from "sonner";
+import { ApiClientError } from "@/lib/api/client";
+
+export type NotifyOptions = {
+  description?: string;
+  duration?: number;
+};
+
+function baseOptions(opts?: NotifyOptions) {
+  return {
+    description: opts?.description,
+    duration: opts?.duration ?? 4000,
+  };
+}
+
+/** Toast notifications — use for transient success/error/info across the app */
+export const notify = {
+  success(message: string, opts?: NotifyOptions) {
+    toast.success(message, baseOptions(opts));
+  },
+  error(message: string, opts?: NotifyOptions) {
+    toast.error(message, { ...baseOptions(opts), duration: opts?.duration ?? 5500 });
+  },
+  warning(message: string, opts?: NotifyOptions) {
+    toast.warning(message, baseOptions(opts));
+  },
+  info(message: string, opts?: NotifyOptions) {
+    toast.info(message, baseOptions(opts));
+  },
+  loading(message: string) {
+    return toast.loading(message);
+  },
+  dismiss(id?: string | number) {
+    toast.dismiss(id);
+  },
+  promise<T>(
+    promise: Promise<T>,
+    messages: { loading: string; success: string; error?: string }
+  ) {
+    return toast.promise(promise, {
+      loading: messages.loading,
+      success: messages.success,
+      error: messages.error ?? messages.loading,
+    });
+  },
+};
+
+export function getErrorMessage(err: unknown, fallback: string): string {
+  if (err instanceof ApiClientError) return err.message;
+  if (err instanceof Error && err.message) return err.message;
+  return fallback;
+}
+
+export function notifyError(err: unknown, fallback: string) {
+  notify.error(getErrorMessage(err, fallback));
+}