refactor(modules): rename system/ to nixos/

Clearer naming to distinguish NixOS system modules from
Home Manager modules (nixos/ vs home/).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

modules/nixos/backlight.nix

@@ -0,0 +1,12 @@
+{
+  pkgs,
+  myConfig,
+  config,
+  ...
+}: {
+  hardware.i2c.enable = true;
+  # boot.extraModulePackages = [config.boot.kernelPackages.ddcci-driver];
+  # boot.kernelModules = ["ddcci_backlight"];
+  environment.systemPackages = with pkgs; [ddcutil];
+  users.users.${myConfig.nixos.username}.extraGroups = ["i2c"];
+}