Address the 11 October test report findings

The initial test report (run before the pilot-readiness changes) raised
seven findings and a retest plan. This closes the ones still open.

- F-01: a business on its free trial may run 2 active branches whatever
  its tier, so an owner can try transfers and per-branch stock. Only
  active branches count towards any limit.
- F-06: owners can rename branches and deactivate/reactivate them from
  the Branches page. A branch can only be deactivated once it holds no
  stock and no active staff; its history stays in every report, and it
  leaves every picker.
- F-04: the Products page lists deactivated products under a toggle with
  Reactivate. Deactivating warns about stock still on hand, which stays
  on the Stock page so it can be transferred or counted out.
- F-05: products carry a unit (L, ml, kg, g, pcs, tray, pack) instead of
  everything being litres.
- CAT-12: product prices must be above zero and names are unique per
  business, ignoring case.
- F-03: a reversed report period is refused by the page and the API, and
  the page labels which period the cards show. Report dates are Nairobi
  business days.
- F-07: cost of goods sold is units sold x average buying price. Stock
  bought in the period is reported separately, with a cash surplus line,
  so unsold stock no longer shows as a loss.
- STA-04: staff and new branch managers can be invited with a one-time,
  7-day link; the person sets their own password. Managers can issue a
  new link for their own cashiers.
- OFF-02 / POS-02: the till shows each product's stock and deducts every
  sale immediately, online or offline; it warns before selling more than
  the recorded stock, and has a Clear sale button.
- npm run backfill:stock turns purchases recorded before the stock ledger
  into stock, so existing tenants (and the report's ZERRO fixtures) start
  from what they bought.

tests/retest-plan.test.ts runs the report's completion and retest plan
and checks every balance and total it predicts.

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

src/lib/sessionCache.ts

@@ -9,10 +9,17 @@ import type { AuthUser, Product } from "@/lib/types";
 
 const KEY = "aquaflow_terminal_session";
 
+/** The branch's stock as the server last reported it. */
+export interface StockSnapshot {
+  byProduct: Record<string, number>;
+  fetchedAt: string;
+}
+
 export interface SessionSnapshot {
   user: AuthUser;
   features: { mpesa: boolean };
   products: Product[];
+  stock?: StockSnapshot;
   savedAt: string;
 }
 
@@ -32,6 +39,7 @@ export function saveSession(update: Partial<SessionSnapshot> & { user: AuthUser
     const next: SessionSnapshot = {
       features: update.features ?? (sameUser ? prev!.features : { mpesa: false }),
       products: update.products ?? (sameUser ? prev!.products : []),
+      stock: update.stock ?? (sameUser ? prev!.stock : undefined),
       user: update.user,
       savedAt: new Date().toISOString(),
     };