Export settings and integrate them with home-manager

[xaltsc]

This solves a part of #559.

This is blocked by #566 #528 #554 #527 #532 #515 and I think that's it.

This PR does three things:

1. Update the rust code so that it can export a schema of the settings in jfn_config::SettingsData through a hidden cli option --generate-settings-schema
2. Create a CI workflow to update those schemas on push. I had to make a weird decision and put this as a job in build-nix.yml so that it would run after the compilation. I'm not sure this is needed though, maybe cachix would catch on the result of the compilation needed for an ab initio nix run ., making the build useless.
3. Handle the schema in nix as a base for the programs.jellyfin-desktop.settings.* options.

As usual, a lot of this needs rebasing, some values pertaining to the cache are placeholder values because #542 is not closed.

The three relevant commits are (by name, since their hash will change as PRs get merged):

1. Add an option to export settings jellyfin/jellyfin-desktop@c88bde0
2. flake: use the generated settings schema to produce options jellyfin/jellyfin-desktop@f6efed6
3. flake+ci: add regen settings job to build-nix workflow jellyfin/jellyfin-desktop@297de8e

Status:

• the cli option works
• the CI job works Update settings schema xaltsc/jellyfin-desktop#14 https://github.com/xaltsc/jellyfin-desktop/actions/runs/28557323106/job/84667538529
• the home-manager module has the correct settings options

[andrewrabert]

@xaltsc please combine the nix PR's and I'll marge asap