Sales: add-item quantity field doesn't make clear which unit it expects — users type egg count instead of pack-unit count #445

Problem

When adding a line item to a sales order, users sometimes type the total egg count into the quantity field instead of the count in the selected pack unit — e.g. selling 2 trays (= 60 eggs), the user types 60 instead of 2. The backend takes the quantity at face value in whatever unit is selected, so this silently records a sale 30x larger than intended (2 trays vs. 60 trays = 1,800 eggs).

Why it happens (confirmed in code)

• Add-item form (web/src/routes/SalesPage.tsx:539-563): a product <select> (options show only p.name, no unit hint — line 547), a separate "Per" unit <select> (line 550-554, e.g. Egg/Dozen/Tray/Case), and the quantity NumberField (line 559-563) labeled with the bare word "Quantity" (web/src/i18n/en.ts:259). Nothing next to the quantity field echoes the selected unit or previews the resulting egg count while typing.
• Once a line is added, the table row does show this info — perUnit/eggsCount render "per tray (30 eggs)" (SalesPage.tsx:480-481) — but only after the fact, too late to catch the mistake.
• Backend takes quantity literally in the selected unit: AddOrderItemHandler.cs:46-53,88-90 resolves the unit's EggUnitConversion and passes command.Quantity straight through; SalesOrder.cs:43-45,58-60 computes quantityBase (eggs) = quantity × baseUnitFactor. So "60" typed against a Tray product (baseUnitFactor=30) becomes 1,800 eggs, not 60 — exactly the reported bug.
• A fix pattern already exists elsewhere in the app and is unused here: quantityLabelWithUnit = "Quantity ({{unit}})" (web/src/i18n/en.ts:540,710) is already used on Inventory (InventoryPage.tsx:444,492,528) and Water (WaterPage.tsx:242), but not on the Sales add-item form.

Suggested UI fix

1. Label the quantity field with its unit, reusing the existing quantityLabelWithUnit pattern instead of the bare t("quantity") at SalesPage.tsx:560-561 — e.g. "Quantity (tray)".
2. Show a live conversion hint next to the quantity field as the user types — "= {quantity × eggsPerUnit} eggs" — computed client-side from the same conversions data already used for the perUnit/eggsCount display at SalesPage.tsx:480-481. This is the most direct fix: the user sees immediately that typing 60 means 1,800 eggs and self-corrects.
3. Annotate the product picker (SalesPage.tsx:547) with the product's default unit size, e.g. "Large Tray (30 eggs/tray)", so the unit is visible before quantity entry even starts.

All three are additive, client-side-only display changes — no backend/domain change needed, since the unit semantics are already correct; the problem is purely that the UI doesn't surface them at entry time.

Docs

Touches user-visible Sales behavior — update specs/product/GLOSSARY.md if the "per-unit quantity" concept isn't already documented there, and the SPA Help page/in-app glossary, per AGENTS.md's doc-sync rule, once implemented.