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/components/ServiceWorkerRegister.tsx

@@ -0,0 +1,18 @@
+"use client";
+
+import { useEffect } from "react";
+
+/**
+ * Registers public/sw.js so the terminal can open offline. Production only:
+ * in `next dev` the build assets are not content-hashed, and caching them
+ * would serve stale code while developing.
+ */
+export default function ServiceWorkerRegister() {
+  useEffect(() => {
+    if (process.env.NODE_ENV !== "production" || !("serviceWorker" in navigator)) return;
+    navigator.serviceWorker.register("/sw.js").catch(() => {
+      // Unsupported or blocked: the app still works online.
+    });
+  }, []);
+  return null;
+}