The deploy form's cluster select could open blank even when a valid
(default) cluster exists for the current scope. The open handler seeds
the cluster once, but that seed can be empty (options not loaded yet) or
point outside the resolved scope (the scope id and the option list both
settle after mount, and switching scope re-scopes the list), and nothing
re-picked afterwards — so the field stayed empty.
Make the form enforce the invariant directly: whenever the scope or the
available options change, if the current selection is empty or not among
the options, fall back to the scope's default cluster (then a Ready one,
then the first). A still-valid selection is left untouched.
