Fix deprecated NixOS options for 25.11

- Use nixpkgs.hostPlatform module instead of deprecated system arg to lib.nixosSystem
- Rename services.logind.powerKey to services.logind.settings.Login.HandlePowerKey

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

flake.nix

@@ -69,9 +69,9 @@
       "straylight"
       "vade"
     ] (hostName: lib.nixosSystem {
-      system = "x86_64-linux";
       specialArgs = { inherit inputs; };
       modules = [
+        { nixpkgs.hostPlatform = "x86_64-linux"; }
         { networking.hostName = hostName; }
         inputs.disko.nixosModules.disko
         ./hw/${hostName}