Tax amount displayed in cart matches final checkout amount

When the cart page computes tax inline as subtotal * 0.085 while the checkout page calls getTaxAmount(), and the order creation API passes a different address to a third calculation, a customer who sees $8.50 in the cart arrives at checkout and sees $8.75 — a difference that reads as a pricing error and triggers abandonment or disputes. CWE-682 (Incorrect Calculation) applies when multiple call sites produce divergent values for the same input. ISO 25010:2011 functional correctness requires that the same cart contents and address produce the same tax amount at every display point. Inconsistency is especially damaging during the payment step: customers read the discrepancy as evidence the system is untrustworthy.

Consolidate all tax display call sites onto a single getTaxAmount function in lib/tax.ts:

// lib/tax.ts — single source of truth, called from all pages
export function getTaxAmount(
  items: OrderItem[],
  address: Address,
  customerIsExempt = false
): number {
  if (customerIsExempt) return 0
  const taxable = items
    .filter(i => !i.is_tax_exempt)
    .reduce((sum, i) => sum + i.price * i.quantity, 0)
  const rate = getTaxRate(address)
  return Math.round(taxable * rate)
}

// Usage on cart page:
const tax = getTaxAmount(cart.items, shippingAddress, user?.is_tax_exempt)
// Same call on checkout page:
const tax = getTaxAmount(cart.items, shippingAddress, user?.is_tax_exempt)
// Same call in order creation API route:
const tax = getTaxAmount(items, address, customer.is_tax_exempt)

Delete any inline subtotal * constant tax expressions scattered across component files and replace with this import. Confirm in a unit test that all three call sites return identical values for the same inputs.