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/kvm.go

@@ -338,20 +338,6 @@ func (s *Server) listKVMs(w http.ResponseWriter, _ *http.Request) {
 	jsonOK(w, 200, out)
 }
 
-// kvmForMachine is the KVM attached to a machine, "" being the hub itself.
-func (s *Server) kvmForMachine(machine string) (config.KVM, bool) {
-	var found config.KVM
-	ok := false
-	s.cfg.View(func(c *config.Config) {
-		for _, k := range c.KVMs {
-			if k.Machine == machine {
-				found, ok = k, true
-			}
-		}
-	})
-	return found, ok
-}
-
 func (s *Server) addKVM(w http.ResponseWriter, r *http.Request) {
 	var body struct{ Name, URL, Machine string }
 	if err := json.NewDecoder(io.LimitReader(r.Body, 4096)).Decode(&body); err != nil {