Cart displays accurate item count and subtotal

A cart that displays the wrong item count or a subtotal that double-counts tax erodes purchase confidence at the exact moment it matters most. ISO 25010:2011 functional correctness requires that computed outputs match specifications — a subtotal inflated by tax inclusion makes the cart total contradict the checkout total, producing customer support tickets, chargebacks, and cart abandonment. CWE-20 (Improper Input Validation) also applies when reduction logic operates on unvalidated price data that can produce negative or overflow subtotals.

Fix the total calculation in src/components/CartSummary.tsx to derive count from quantity sums and subtotal from line item products only — never include tax in the reduce:

// src/components/CartSummary.tsx
const itemCount = items.reduce((sum, item) => sum + item.quantity, 0)
const subtotal = items.reduce((sum, item) => sum + (item.price * item.quantity), 0)

If totals are displayed in more than one location (header badge, drawer, cart page), drive all of them from a single shared selector or context value so a quantity update in one location propagates everywhere without a page reload.