add flatpak to electron build list

[youngcw]

This adds flatpak to the list of electron build formats. The hope is that we can publish to flathub as a linux equivalent to the mac or windows store. I don't know if we can publish to flathub directly with this build process or if we would need to have a separate workflow to generate the package.

Notes:

• I built and installed locally and so far things seem to work fine functionally.
• At least on my setup the menu bar at the top doesn't match the system theme the same way the app image does, but that might be an issue on my end not the app itself.
• I cant find a way to attach a version to the build so it shows up as an unknown version, maybe that only shows up if published.
• For now both the appimage and this get zipped together. Maybe ill try to make a separate runner for each type

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Bundle Stats — desktop-client

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9	4.65 MB	0%

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Asset	File Size	% Changed
static/js/indexeddb-main-thread-worker-e59fee74.js	13.5 kB	0%
static/js/resize-observer.js	18.37 kB	0%
static/js/BackgroundImage.js	122.29 kB	0%
static/js/usePreviewTransactions.js	790 B	0%
static/js/AppliedFilters.js	20.35 kB	0%
static/js/narrow.js	58.84 kB	0%
static/js/wide.js	264.06 kB	0%
static/js/ReportRouter.js	1.19 MB	0%
static/js/index.js	2.97 MB	0%

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kcab.worker.js	1.2 MB	0%