Fix instance types and overrides UI states

Found while preparing the GCP Windows repro for Task 9082 on dev (Products > Instance types; Customers > Operator > Overrides). - Products > Instance types > Browse: after switching Cloud from AWS to GCP, the Region select keeps showing AWS regions (us-east-1…) and the list shows skeletons until the GCP connection's regions load. Clear the region on a cloud change and list only that cloud's regions. - Customers > <org> > Overrides: pressing Save shows "Saving…" while a "Save overrides?" confirm dialog waits for Apply, and navigating away raises a browser "leave page" prompt. While the confirm is open the button should say Save, not Saving…. Out of scope: any change to what the overrides or instance types do. Done means: both flows checked on a local stack with a screenshot each; the web unit tests for the two pages are green.

Fixed Browse: a cloud switch no longer shows the previous cloud's region/fetched line; Overrides Save reads "Save" (disabled) behind its confirm. Tests added, quick gate green, visual check done on local stack. PR #6671 queued to the merge lane (deploy watch left to the lane).