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/server/models/Product.ts

@@ -4,8 +4,10 @@ export interface IProduct extends Document {
   _id: Types.ObjectId;
   businessId: Types.ObjectId; // tenant this product belongs to
   label: string;
+  // Size measured in `unit` (litres for water, a count for side items).
+  // The field keeps its historical name so existing data needs no migration.
   sizeLiters: number;
-  unit: string; // display unit, e.g. "L" for water, "pcs" / "tray" for eggs
+  unit: string; // see PRODUCT_UNITS in src/lib/units.ts: "L", "pcs", "tray"...
   category: string; // e.g. "Water", "Eggs"
   unitPrice: number;
   sortOrder: number;