mpd: set network settings as environment variables

https://mpd.readthedocs.io/en/stable/client.html#connecting-to-mpd

These environment variables are picked up by, amongst others, clients
based on libmpdclient:

- 2a42a10067/src/settings.c (L108)
- 2a42a10067/src/settings.c (L135)

modules/misc/news/2026/01/2026-01-02_00-03-48.nix

@@ -0,0 +1,10 @@
+{ config, ... }:
+{
+  time = "2026-01-02T00:03:48+00:00";
+  condition = config.services.mpd.enable;
+  message = ''
+    `MPD_HOST` and `MPD_PORT` environment variables are now set automatically.
+
+    This can be disabled with `services.mpd.enableSessionVariables = false`.
+  '';
+}

modules/services/mpd.nix

@@ -22,6 +22,15 @@ in
 
       package = lib.mkPackageOption pkgs "mpd" { };
 
+      enableSessionVariables = mkOption {
+        type = types.bool;
+        default = true;
+        description = ''
+          Whether to set {env}`MPD_HOST` {env}`MPD_PORT` environment variables
+          according to {option}`services.mpd.network`.
+        '';
+      };
+
       musicDirectory = mkOption {
         type = with types; either path str;
         defaultText = lib.literalExpression ''
@@ -155,7 +164,13 @@ in
       );
     in
     mkIf cfg.enable {
-      home.packages = [ cfg.package ];
+      home = {
+        packages = [ cfg.package ];
+        sessionVariables = mkIf cfg.enableSessionVariables {
+          MPD_HOST = mkIf (cfg.network.listenAddress != "any") cfg.network.listenAddress;
+          MPD_PORT = builtins.toString cfg.network.port;
+        };
+      };
 
       services.mpd = lib.mkMerge [
         (mkIf (lib.versionAtLeast config.home.stateVersion "22.11" && config.xdg.userDirs.enable) {