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/admin/ActivityFeedMatrix.tsx

@@ -45,7 +45,7 @@ export default function ActivityFeedMatrix({ transactions, branchNames, connecte
             <li key={t._id} className="flex items-center justify-between gap-3 px-4 py-3 animate-fade-up">
               <div className="min-w-0">
                 <div className="flex items-center gap-2">
-                  <p className="truncate text-sm font-medium text-depth-900">{t.customerName || t.customerPhone}</p>
+                  <p className="truncate text-sm font-medium text-depth-900">{t.customerName || t.customerPhone || "Walk-in customer"}</p>
                   {scope === "global" && (
                     <span className="whitespace-nowrap rounded-full bg-depth-100 px-2 py-0.5 text-[10px] font-semibold uppercase tracking-wide text-depth-600">
                       {branchLabel}