System > Sources & scans renders the concept's section on the server's
sources: one row per effective source with its watch state and a scan
scoped to that source, the 3.2.0 update notice with the legacy dismissal,
Add source checked by validate-source, Remove, monitoring and worker
threads as host drafts behind the one Save row, and Refresh files.
A save refused by a running scan names the job. Thumbnail generation
hands off to the legacy gallery until S5.
Deviations D98, D113 and D-NEW-S4-1 are recorded in TESTING.md.