chore: add files to manage flathub details upstream (#104)

* Add files for flathub compliance

Please see: https://github.com/flathub/flathub/pull/7783

This adds the requested files to the upstream git repo, and adds the
screenshot.png so it may be sourced from Stoat, rather than from a user
repository.

* chore: update product screenshot

* chore: update desktop entry comment

* chore: update icon

* chore: update summary to current github tag

---------

Co-authored-by: Shawn W Dunn <sfalken@opensuse.org>
Co-authored-by: izzy <me@insrt.uk>

chat.stoat.StoatDesktop.desktop

@@ -0,0 +1,11 @@
+[Desktop Entry]
+Name=Stoat
+Comment=Open source, user-first chat platform
+Exec=stoat-desktop
+Terminal=false
+Type=Application
+Icon=chat.stoat.StoatDesktop
+Categories=Network;InstantMessaging
+StartupWMClass=stoat-desktop
+X-Desktop-File-Install-Version=0.26
+X-Flatpak=chat.stoat.StoatDesktop