Add linting, formatting, and CI

- Add deadnix, nixfmt, and statix to flake inputs
- Add formatter output to flake for nix fmt support
- Add deadnix, nixfmt, statix to Home Manager packages
- Format all nix files with nixfmt
- Add GitHub Actions workflow for CI validation
- Support x86_64-darwin in formatter

modules/shared/nixos/flatpaks.nix

@@ -1,5 +1,5 @@
-
-{ ... }: {
+{ ... }:
+{
   services = {
     flatpak = {
       enable = true;