Applications menu for KDE apps in the Steam session

SteamOS only ships /etc/xdg/menus/plasma-applications.menu; the nested
desktop sets XDG_MENU_PREFIX=plasma-, the Steam session doesn't, so KDE
apps started there (Dolphin from the "+" menu) built an empty app
database: "no installed application can open" every file.

session.applicationsMenu (module applications-menu, on by default) links
~/.config/menus/applications.menu to Plasma's menu. Flake check: the link
is generated by default and absent when disabled.

modules/applications-menu.nix

@@ -0,0 +1,27 @@
+# Workaround (SteamOS 0.3.0, build 20260922): KDE apps build their app
+# database (ksycoca) from "applications.menu", but SteamOS only ships
+# /etc/xdg/menus/plasma-applications.menu. The nested desktop sets
+# XDG_MENU_PREFIX=plasma-, the Steam session doesn't, so KDE apps started
+# there (Dolphin from the "+" menu) know no apps. We link the user's
+# applications.menu to Plasma's. A missing host file leaves a dangling link,
+# which KDE treats like no file. Remove once the Steam session sets the prefix.
+{ config, lib, ... }:
+let
+  sys = "/etc/xdg/menus/plasma-applications.menu";
+in {
+  imports = [ ./cleanup.nix ];
+
+  options.steamFrame.session.applicationsMenu.enable = lib.mkOption {
+    type = lib.types.bool;
+    default = true;
+    description = ''
+      Link `~/.config/menus/applications.menu` to Plasma's menu, so KDE apps
+      in the Steam session find the installed apps.
+    '';
+  };
+
+  config = lib.mkIf config.steamFrame.session.applicationsMenu.enable {
+    xdg.configFile."menus/applications.menu".source =
+      config.lib.file.mkOutOfStoreSymlink sys;
+  };
+}