Make the terminal work offline end to end, and add stock screens

Terminal (PRD 5.3/5.5):
- Opens with no network: a service worker caches the app shell and build
  assets, and the last confirmed session and product list are kept on the
  phone. Only a definitive 401/403 sends the cashier to the login page.
- The offline queue is keyed by cashier and branch, sends only the
  signed-in cashier's sales, chunks to the server's batch limit, and acts
  on the per-sale result: synced, or rejected with the reason shown and
  never retried. Rows from the old queue are adopted on first sync.
- Several products per sale (tap to add), optional customer details, and
  a printable receipt. The banner shows sales waiting to sync and any
  that need attention; sign-out warns about unsynced sales.
- M-Pesa only appears when FEATURE_MPESA is on. A timed-out prompt now
  tells the cashier not to take cash until the payment is checked,
  instead of reporting it as failed.

Admin:
- New Stock page: stock per branch with low/out flags, physical counts
  and adjustments, inter-branch transfers, morning dispatch and evening
  count entry, and recent movements.
- Dashboard shows stock on hand per branch and links to Stock.
- Staff page can deactivate and reactivate accounts.
- Products page is read-only for branch managers and edits the low-stock
  alert level.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/layout.tsx

@@ -1,6 +1,7 @@
 import type { Metadata, Viewport } from "next";
 import "./globals.css";
 import { ToastProvider } from "@/components/ui/Toast";
+import ServiceWorkerRegister from "@/components/ServiceWorkerRegister";
 
 // NOTE: We deliberately do NOT use `next/font/google` here. That loader
 // downloads the font files from Google at *build time*, which fails in
@@ -34,6 +35,7 @@ export default function RootLayout({ children }: { children: React.ReactNode })
         />
       </head>
       <body>
+        <ServiceWorkerRegister />
         <ToastProvider>{children}</ToastProvider>
       </body>
     </html>