refactor: switch to home-manager module

Systant is a userspace controller, so it makes sense to manage it
via home-manager rather than as a system service. This allows:
- Declarative per-user configuration
- Access to user's environment, PATH, and session
- Proper handling of audio, display, and other user resources

Usage in home-manager config:
  imports = [ inputs.systant.homeManagerModules.default ];
  services.systant.enable = true;
  services.systant.settings = { ... };

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

flake.nix

@@ -12,8 +12,8 @@
       ...
     }:
     {
-      # NixOS module (system-independent)
-      nixosModules.default = import ./nix/nixos-module.nix;
+      # Home Manager module (system-independent)
+      homeManagerModules.default = import ./nix/nixos-module.nix;
 
       # Overlay to add systant to pkgs
       overlays.default = final: prev: {

nix/nixos-module.nix

@@ -1,7 +1,7 @@
 { config, lib, pkgs, ... }:
 
 let
-  cfg = config.systant;
+  cfg = config.services.systant;
   settingsFormat = pkgs.formats.toml { };
   configFile =
     if cfg.configFile != null
@@ -11,7 +11,7 @@ let
     else null;
 in
 {
-  options.systant = {
+  options.services.systant = {
     enable = lib.mkEnableOption "systant system monitoring agent";
 
     package = lib.mkOption {
@@ -62,17 +62,16 @@ in
   };
 
   config = lib.mkIf cfg.enable {
-    # Make the package available system-wide
-    environment.systemPackages = [ cfg.package ];
+    home.packages = [ cfg.package ];
 
-    # Systemd user service - runs in user session with user's environment
     systemd.user.services.systant = {
-      description = "Systant system monitoring agent";
-      wantedBy = [ "default.target" ];
-      after = [ "network-online.target" ];
-      wants = [ "network-online.target" ];
+      Unit = {
+        Description = "Systant system monitoring agent";
+        After = [ "network-online.target" ];
+        Wants = [ "network-online.target" ];
+      };
 
-      serviceConfig = {
+      Service = {
         Type = "simple";
         ExecStart =
           if configFile != null
@@ -81,6 +80,10 @@ in
         Restart = "on-failure";
         RestartSec = "5s";
       };
+
+      Install = {
+        WantedBy = [ "default.target" ];
+      };
     };
   };
 }