I guess I'll chime in on this since I love electronics and hobby projects are always always fun to look at.
So i am going to start with the good:
This is nice idea it really is, if you don't have a smart phone, or a computer handy at the time this works nicely. It's simple with a single button operation and it seems to just work, also I like the fact that it's a custom enclosure too. I can see the effort you put into making this and applaud you of you work. I love microcontrollers and wifi (IoT stuff lol) so I enjoy the use of it in this project (but then against how else would you do without it?) This fills a niche need and sometimes things like this are wanted / needed. Good job.
So the Bad (in my humble opinion):
I will not re-iterate what everyone else has said but instead let's take a look at your project. The PCB design isn't bad but it kinda reminds me of a auto routed circuit, maybe it is hand routed and I am mistaken but some of your traces could be cleaned up. A couple of them have a small zig-zag in them or are very very close together, while this probably will not affect operation at all since how wide the traces are it's just something to consider. Honestly it's fine, i'm just being nit picky.
When you are taking photos of your project and you have a very reflective surface, consider not using flash. Try using a tripod to allow for ambient lighting, as it hurts to look at the beautiful PCB board you made in that picture.
Why other are saying what they are:
A couple other members have commented on phones and even specialized wifi SD cards. While they are not trying to say your work is bad they are just pointing out common options that many have access to. They aren't ridiculing your work just questioning the need for such a device which you have stated. Here on the forum we trade a lot of ideas around as you'll notice that they are praising your work but still feel there is another way to solve this issue. I agree there isn't just one way to solve an open problem like this.
Closing thoughts:
This again is a really cool project, I'd update the PCB board and make this really modular. What if instead of a single use application then we can use this for other things, because truely it is a mini webserver. Why not add headers to both controllers and have them removable so people can play and tinker around with the Atmega328p and wifi controller on their own? So now they are not just buying a single use thing but can expand on your ideas if gateway later on comes up with a new solution that no longer requires this. If your goal is really single application I think this can be shrunk down immensely, for LiPo charger you could use a TP4056, maybe use an ATTiny, and your wifi module but stack them to save space. Either way I am happy I got to see this and thank you for sharing