fix Nix package dependencies

[wojcik91]

Fix missing runtime dependencies, bundle resources (icons etc), get the NixOS module to actually work.

The NixOS module is still a bit WIP. I'm currently using it but there are apparently still some issues with resolvconf (used by the background service to configure DNS if provided) in some setups. It works for me with networking.networkmanager.enable = true; but apparently services.resolved has to be disabled.

To use the module you can add this repo to your flake inputs:

# Defguard desktop client
    defguard-client = {
      # can't use default github schema due to https://github.com/NixOS/nix/issues/13571
      url = "git+https://github.com/DefGuard/client?ref=refs/tags/v1.6.2-nixtest3";
      flake = true;
      inputs.nixpkgs.follows = "nixpkgs";
      inputs.flake-utils.follows = "flake-utils";
    };

And then add this somewhere in your NixOS config:

  imports = [
    inputs.defguard-client.nixosModules.default
  ];

  # add user to defguard group for socket access
  users.users."yourUserName".extraGroups = ["defguard"];

  programs.defguard-client.enable = true;

Huge thanks to @kristofvandam for contributing a fix for missing runtime dependencies: #741

Closes #728