Enable Open WebUI as system module

Add flatpak identifier for Open WebUI, move its enable flag from home
modules to system modules, and remove the corresponding XDG desktop
entry.

modules/core/flatpak.nix

@@ -16,6 +16,7 @@
     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";