Feat: Flatpak Build

[leherkom]

Pull request

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Please describe your changes

Adds the required manifests and metadata to build a Flatpak and possibly publish it to Flathub so it can be installed via app stores that refer to Flathub (such as Discover). Includes a guide on building the Flatpak locally.

Additional context

There are still some considerations before publishing to Flathub:

• Evidently, the manifest refers to my feature branch right now. This is so the app build works in the first place because the added files would otherwise be missing from the repo copy within the build container. Should be changed to point to main (or a release tag that includes all files) when tested and approved.
• Comments and descriptions in the metadata are just texts that I grabbed off the website. Should likely be reviewed and can be improved upon.
• Pretty sure that the autostart option is not going to work in a sandboxed Flatpak?
• The application's title bar is going to respect the environment's GNOME color settings and might show up in light mode even when a dark theme is used in Kanri (and vice versa). I have not yet found a fix for this other than moving from the native title bar to a custom one.

When all is done, submission to Flathub can be done as described here: https://docs.flathub.org/docs/for-app-authors/submission#submission-pr

Fixes #739