change(nix): make flake more consistent with nixos module

[yipfluoric]

Summary

Switching programs.noctalia-greeter to services.displayManager.noctalia-greeter for sanity when switching between nixpkgs and flake.

Motivation

In #3, keeping it as programs.noctalia-greeter was fine, since it was insanely early. Now that the package is in nixpkgs and most people use that variation, using the flake module can conflict with services.displayManager.noctalia-greeter and be a nuisance until you change it to programs.noctalia-greeter. This changes it to the terminology nixpkgs uses for sanity.

Type of Change

• Bug fix
• New feature
• Breaking change
• Refactoring
• Build / packaging

Related Issue

Testing

nh os switch -u
Mostly just testing using my repo rather than noctalia one in my flake

Manual Coverage

• Tested under greetd (real login flow)
• Tested with just run / just run-local (dev compositor)
• Tested with multiple monitors
• Tested appearance sync from Noctalia Shell (Sync Now)
• Tested on NixOS (programs.noctalia-greeter)
• Tested with a pinned [output].name
• Tested with custom [output].layout / [output].transforms

Screenshots / Videos

Checklist

• This PR is ready for review, or it is marked as Draft.
• I read and followed the relevant guidance in CONTRIBUTING.md and README.md.
• I ran just format with clang-format v22+ installed, or this PR has no code changes.
• I ran the relevant build or test commands, or explained why they were not run.
• I self-reviewed the changes.
• I checked for new warnings or errors.
• I updated user-facing documentation in docs/user/ when this PR changes user-visible behavior or configuration, or this PR does not require documentation changes.
• I used the existing canonical names for config keys, paths, and identifiers.

Additional Notes

[github-actions]

The description now contains the required template structure.