Improve PR diff view on mobile (#27883)

Improve PR diff view on mobile (#27883)

1. Show diff stats only on large screens

these are already shown in tabs, so no need for this duplicate
information on small screens

![image](https://github.com/go-gitea/gitea/assets/1135157/1287839d-7490-42eb-a17e-d526dc0bfd9e)

![image](https://github.com/go-gitea/gitea/assets/1135157/e9dcd89d-ed4d-4945-a7aa-4e6fc6d9c3a2)

2. Hide viewed files information on small screens

Github does the same and this gives us more free space on small screens

![image](https://github.com/go-gitea/gitea/assets/1135157/e90b042f-fffb-4f79-a5ae-cd480c9d8334)

![image](https://github.com/go-gitea/gitea/assets/1135157/d2480ffe-58f2-4694-8ae1-a2ab0aae14d4)

3. Review bar now doesn't wrap so we don't need the 77px even on very
small screens

(the sticky headers are still working)

![image](https://github.com/go-gitea/gitea/assets/1135157/42b19b2b-73ef-4b88-8680-c555879b363b)