Better appstream screenshots

The app listing passes all Flathub metadata checks except one: The window buttons are not using the platform default (close-only, no minimize). Would be great to fix this when you next re-take the screenshots, so the app can be featured on Flathub :)

Start by reproducing the Flathub metadata check and inspect how the app window controls are configured before retaking the app listing screenshots. Done means the screenshots show close-only platform-default window buttons and the listing passes all Flathub metadata checks.

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