feat: port to gnome 46 runtime

[Hernz-bearnz]

Feature description

After running a flatpak update, I received a notice stating the following
Info: runtime org.gnome.Platform branch 44 is end-of-life, with reason: The GNOME 44 runtime is no longer supported as of March 20, 2024. Please ask your application developer to migrate to a supported platform. Info: applications using this runtime: com.github.GradienceTeam.Gradience
I have also noticed that themes do not seem to be applying. The out of date runtime could potentially be causing issues for newer versions of GTK.

Additional Context

OS: Fedora 40 workstation edition.
App version: 0.4.1

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• I have read the Code of Conduct.

[github-actions]

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