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/manifest.ts

@@ -0,0 +1,13 @@
+import type { MetadataRoute } from "next";
+
+// Lets cashiers add the terminal to their home screen and open it full-screen.
+export default function manifest(): MetadataRoute.Manifest {
+  return {
+    name: "Vessel Branch Terminal",
+    short_name: "Vessel",
+    start_url: "/terminal",
+    display: "standalone",
+    background_color: "#F5F2EF",
+    theme_color: "#0C0C0C",
+  };
+}