A sweep of the whole console: status codes, a data race, atomic writes, dead code

From the race detector, staticcheck, a garbage-in battery over every safe route and a
headless pass through every tab on two machines. Fixed:
- docker inspect/health of a container that isn't there answered 500; now 404, and a
  daemon that is down 503 (dockerErr, with a test).
- /api/virt/instances/{name} on a machine without Incus answered 502 "incus not found";
  it now says Incus isn't ready, like its siblings.
- Deleting a ZFS snapshot with a bad name answered 500; a bad name is 400, a snapshot
  that doesn't exist 404.
- The task list read a task's Running flag without the task's lock while the waiter
  goroutine cleared it: a data race, now read under the lock.
- The Virtualization export config and run record were written in place; a crash
  mid-write would have left a half file. Atomic now, like the config and state files.
- An unused function in kvm.go, one in incus/subnet.go, an unused field in files/move.go
  and a capitalised error string, from staticcheck.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

internal/api/zfs.go

@@ -265,7 +265,15 @@ func (s *Server) zfsSnapshotDelete(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	if err := s.storage.DestroySnapshot(r.Context(), body.Name, body.Recursive); err != nil {
-		jsonErr(w, 500, err.Error())
+		msg := err.Error()
+		switch {
+		case msg == "bad snapshot name":
+			jsonErr(w, 400, msg)
+		case strings.Contains(msg, "does not exist"):
+			jsonErr(w, 404, msg)
+		default:
+			jsonErr(w, 500, msg)
+		}
 		return
 	}
 	log.Printf("zfs snapshot %s destroyed by %s%s", body.Name, clientIP(r), map[bool]string{true: " (recursive)", false: ""}[body.Recursive])