refactor: HostedCart component

[acasazza]

Summary

Refactor HostedCart.tsx to fix bugs and align with the patterns established in CheckoutLink and CartLink.

Changes

• Fix Rules of Hooks violation — move all useContext calls above the early return null
• Update JSDoc — replace deprecated <OrderContainer> with <Order>
• Extract duplicated domain constant — 'commercelayer.io' hardcoded twice
• Fix missing customDomain — second getApplicationLink call inside useEffect was not forwarding the prop
• Extract shared cart URL resolver — deduplicate getOrganizationConfig + getApplicationLink logic
• Fix useEffect dependency anti-pattern — [ref.current != null] is not a valid dep array for iframe resizer setup
• Replace .then() with async/await in the effect
• Remove commented-out code — // zIndex lines in default styles