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/lib/sessionCache.ts

@@ -0,0 +1,50 @@
+import type { AuthUser, Product } from "@/lib/types";
+
+/**
+ * The last confirmed session and catalogue, so the terminal can start and
+ * keep selling with no network. Only a snapshot for display and for
+ * labelling queued sales: the server still authenticates every sync with
+ * the httpOnly cookies and re-prices every line.
+ */
+
+const KEY = "aquaflow_terminal_session";
+
+export interface SessionSnapshot {
+  user: AuthUser;
+  features: { mpesa: boolean };
+  products: Product[];
+  savedAt: string;
+}
+
+export function loadSession(): SessionSnapshot | null {
+  try {
+    const raw = localStorage.getItem(KEY);
+    return raw ? (JSON.parse(raw) as SessionSnapshot) : null;
+  } catch {
+    return null;
+  }
+}
+
+export function saveSession(update: Partial<SessionSnapshot> & { user: AuthUser }): void {
+  try {
+    const prev = loadSession();
+    const sameUser = prev?.user.id === update.user.id;
+    const next: SessionSnapshot = {
+      features: update.features ?? (sameUser ? prev!.features : { mpesa: false }),
+      products: update.products ?? (sameUser ? prev!.products : []),
+      user: update.user,
+      savedAt: new Date().toISOString(),
+    };
+    localStorage.setItem(KEY, JSON.stringify(next));
+  } catch {
+    // Storage full or blocked: the terminal still works online.
+  }
+}
+
+export function clearSession(): void {
+  try {
+    localStorage.removeItem(KEY);
+  } catch {
+    // ignore
+  }
+}