Clarify the order of operations in the add order item modal

Describe your bug or feature request.

We’ve discussed holistic changes to the “Add order item” modal workflow, which I think we’d summarize:

Do not float the “Add new order item” button to the right of the search bar. It should also just be labeled “Next”.

If possible via simple JS, let’s allow an “enter” keypress while in the autocomplete textfield to click the “Next” button, so I can just type in my SKU, select it, and hit enter to proceed.

Use a “continue” color (e.g., green) to identify partial steps toward full element submission.

For the next phase, where the quantity textfield and override unit price elements are shown, the button should be labeled “Add” and should also use the green continue color.

The “Remove” operations button for a staged order item should be fixed to just use the secondary button styling. This is not data destructive, just zeroing out staged data, so it does not need red.

When you edit a staged order item, the button text to stage changes should be labeled “Update”.

The “Add items” button should be “Save” and ideally would be disabled until a save would actually result in the creation of new order items. Otherwise, the admin should just cancel.

Switch to a regular outlined button for “Cancel.”