Add Open WebUI desktop entry; enable in home

- Move openWebUI.enable from system-modules to home‑modules for desktop
  and laptop hosts
- Remove flatpak entry for OpenWebUI
- Add XDG desktop entry and bind Super‑Ctrl‑O to launch it via dex
- Update Helium appimage to version 0.14.2.1 with new checksum

modules/core/flatpak.nix

@@ -16,7 +16,6 @@
     flatseal = "com.github.tchx84.Flatseal";
     upscaler = "io.gitlab.theevilskeleton.Upscaler";
     arduino = "cc.arduino.IDE2";
-    openWebUI = "com.openwebui.open-webui";
   };
 in {
   options.flatpak.enable = lib.mkEnableOption "Flatpak";

modules/core/packages/appimages/helium.nix

@@ -4,8 +4,8 @@
   fetchurl,
 }: let
   pname = "helium";
-  version = "0.13.5.1";
-  hash = "sha256-VAncL78nyXRRNUPZ2c0QudxFrfxy7tXE2NFN4teeezk=";
+  version = "0.14.2.1";
+  hash = "sha256-JQQ5K+WUZ3ly5mklNPGZWDTlq+mJqqS02oJgC3in5U0=";
 
   src = fetchurl {
     url = "https://github.com/imputnet/helium-linux/releases/download/${version}/helium-${version}-x86_64.AppImage";