feat(jobs): show background work where the work is started

Reading the app before moving anything turned up that four of the five
job kinds already have a home showing their work: Settings → Search
Index draws per-tier index progress, `downloads-view` draws every
download's lifecycle state, `autotag-view` draws its own apply ring,
and only `library-scan` had nowhere but the Jobs tab. What none of the
four had is the *generic* affordances — pause, cancel, Details, the
log, and a finished job you can dismiss.

So this is a panel embedded beside each of them rather than one
"Background jobs" section in Settings, which would have been the tab
again under another name.

Three rules in it. The controls are `applyJobControl`, not a
reimplementation, which is what keeps the index build's "you will
discard hours of downloading" confirmation alive across the move. A
panel with nothing to say is `hidden` rather than empty, host margin
included, because an idle panel in four places is four pieces of
furniture describing an absence. And there is no "Clear finished":
`ClearFinishedJobs` is global, so a Clear under Libraries would discard
the index build's history too.

`JobKind` also gains `download`, which the backend has had all along.

frontend/src/store/job-store.ts

@@ -30,6 +30,7 @@ export type JobState =
 export type JobKind =
     | 'library-scan'
     | 'index-build'
+    | 'download'
     | 'autotag-apply'
     | 'catalog-enrich';