fix: decrease width-request of main window to properly support Linux mobile

[radmorecameron]

The app is working fairly well on Mobile Linux as is but this should improve the experience (and also include the app is the Mobile Apps/On the go section in Flathub + show that the app is mobile friendly in the Gnome Software Center).

From https://docs.flathub.org/docs/for-app-authors/metainfo-guidelines#desktop-and-mobile-apps :
"Apps that support and work on mobile, tablet and desktop typically should have:

<recommends>
  <control>keyboard</control>
  <control>pointing</control>
  <control>touch</control>
</recommends>
<requires>
  <display_length compare="ge">360</display_length>
</requires>

"

I didn't notice any issues with this change when the width of the app is 360px

[1peter10]

+1 to this, I tried this and did not notice any drawbacks on my laptop.