fix(mail_queue): remove invalid nil check for struct type

QueueConfig is a struct, not a pointer, so comparing to nil is invalid.
The `!ok` check is sufficient for map key existence.

Fixes build error introduced in PR #437.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/cmd/mail_queue.go

@@ -35,7 +35,7 @@ func runMailClaim(cmd *cobra.Command, args []string) error {
 	}
 
 	queueCfg, ok := cfg.Queues[queueName]
-	if !ok || queueCfg == nil {
+	if !ok {
 		return fmt.Errorf("unknown queue: %s", queueName)
 	}