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/incus/subnet.go

@@ -14,7 +14,6 @@ import (
 	"net/netip"
 	"os"
 	"os/exec"
-	"strings"
 	"time"
 )
 
@@ -125,6 +124,3 @@ func dockerPools() []string {
 	}
 	return []string{"172.17.0.0/12", "192.168.0.0/16"}
 }
-
-// String form used in the layout file and on the page.
-func prefixString(p netip.Prefix) string { return strings.TrimSpace(p.Masked().String()) }