[Feature]: Flatpak support #1093

Feature description

Now that we're planning on deprecating AppImage (#1082), we need to support FlatPak as an alternative agnostic distribution mechanism.

Implementation Suggestion

Electron Builder does not currently support publishing to FlatHub. FlatHub only allows packages that they can build on their infra and Electron Builder is not currently capable of supplying a manifest that would allow their infra to build the packages.

At a brief glance, it seems doable, though it is still a lot of legwork to set up. We'd need to pre-fetch all dependencies, including the Electron binaries, place them in a cache, and update electron-builder so that it can find these cached dependencies when run inside FlatHub's builders.

Another option is to copy VSCode's method and ship the entire DEB as an extra-data field and then have the build script just unpack this into the sandbox.

Separately, the permissions story for FlatHub is pretty tricky. We would likely want to add first-run script that would copy WSH into the main filesystem and then WaveSrv can use a socket to communicate with it.

Anything else?

No response