Supervisor partially putting themselves as first author even though I've done most of the work and written most of the paper
I would apply Hanlon's Razor here: "Never attribute to malice that which is adequately explained by stupidity."

This sort of mistake is easy to make when filling out journal forms: there is a lot of information to keep straight.  In particular, the UI is not always designed so as to make it clear how author order will be interpreted (some forms might always want the corresponding author entered first, even if they will not be the first author).  So it's not too hard to believe that someone could innocently mess it up twice.

It would also be an ineffective way for your supervisor to try to "steal" first authorship.  For most journals, if the paper is accepted, every author will eventually be asked to sign a consent form agreeing to publication, and in particular, agreeing to the author ordering.  You'd eventually find out and make a fuss.  

But it would be fine to ask your supervisor to contact the editor and request the change, "just so that we don't forget later," or, as Azor Ahai suggests, to do it yourself.  After all, your supervisor has agreed, in writing, that you are meant to be the first author.


  Addition: I think perhaps the author list inputted in the online fields may be used for citations but the paper itself will have me as first author. So perhaps my supervisor wants their name to be that cited?


No, it doesn't work like that.  The journal is going to want one and only one author ordering, to be used both on the paper itself and for all metadata and citations.  They should always match.  If there is a discrepancy between the manuscript and the online fields, the journal will insist that the authors resolve it before publication.