Implement AbortController-based cancellation for individual search sources
during background resolution. Users can now cancel a single pending/running
source search via a new cancel button in the progress UI, while other sources
continue uninterrupted. Add cancelled status tracking, source-level AbortController
management, CSS styling for the cancelled state, and cleanup on job teardown.
Also refactor getBackgroundSearchJob for consistent job lookup across callers.
Dariusz L
2e9ffb05a8