feat(jobs): register the autotag apply, and ask before quitting

The apply was a bare goroutine whose progress lived in a component
field discarded on navigation, with no cancel and no record of where
it stopped if the app quit while it was rewriting tags — beside a
registry that gives every other long-running operation exactly those
things.

`jobs.KindAutotagApply` now carries progress, a cancel wired to the
apply's context, and a terminal state that tells cancelled from
failed. `OnBeforeClose` returns false unconditionally today; it now
asks while a file-writing job is in flight.

Still not durable: quitting cancels cleanly but nothing records where
it stopped for the next launch. That belongs with the deferred
download/jobs work.

backend/jobs/jobs.go

@@ -22,9 +22,10 @@ type Kind string
 
 // Job kinds.
 const (
-	KindLibraryScan Kind = "library-scan"
-	KindIndexBuild  Kind = "index-build"
-	KindDownload    Kind = "download"
+	KindLibraryScan  Kind = "library-scan"
+	KindIndexBuild   Kind = "index-build"
+	KindDownload     Kind = "download"
+	KindAutotagApply Kind = "autotag-apply"
 )
 
 // State is the lifecycle position of a job.