Refund support

[kilbot]

Support processing refunds directly from the POS. WooCommerce already
exposes a refunds endpoint on orders — we need to wrap that in the
Pro plugin and build a refund flow in the app.

Pro plugin (woocommerce-pos-pro):

• Wrap the WC REST API /orders/{id}/refunds endpoint
• Handle full and partial refunds
• Ensure proper permission checks for POS cashiers

App (monorepo):

• Refund UI (exact placement TBD — could live on the Orders screen,
  or be accessible from a completed cart/order)
• Line-item selection for partial refunds
• Refund reason input
• Confirmation flow

Pro only.