Description of problem:

Currently, widgets require a fetch:response:value selector to set a default value, which means a fetch call must succeed for any default to be applied.There are scenarios where:

• You want a static fallback default when the fetch fails or hasn't completed yet

• You want to pre-populate a field with a known value without requiring a fetch at all

• The fetch depends on other fields (fetch:retrigger) that aren't filled yet, but you still want an initial default

Add a new ui:props option fetch:response:default that provides a static default value applied immediately on widget mount, acting as a fallback when fetch fails or hasn't completed, and gets overridden by fetch:response:value once fetch succeeds.