adding automatic flatpak management

system/extras/flatpak.nix

@@ -0,0 +1,33 @@
+{ config, pkgs, ... }:
+let
+  grep = pkgs.gnugrep;
+  desiredFlatpaks = [
+    "com.spotify.Client"
+    "io.github.huderlem.porymap"
+  ];
+in {
+  system.userActivationScripts.flatpakManagement = {
+    text = ''
+      ${pkgs.flatpak}/bin/flatpak remote-add --if-not-exists flathub \
+        https://flathub.org/repo/flathub.flatpakrepo
+
+      installedFlatpaks=$(${pkgs.flatpak}/bin/flatpak list --app --columns=application)
+
+      for installed in $installedFlatpaks; do
+        if ! echo ${toString desiredFlatpaks} | ${grep}/bin/grep -q $installed; then
+          echo "Removing $installed because it's not in the desiredFlatpaks list."
+          ${pkgs.flatpak}/bin/flatpak uninstall -y --noninteractive $installed
+        fi
+      done
+
+      for app in ${toString desiredFlatpaks}; do
+        echo "Ensuring $app is installed."
+        ${pkgs.flatpak}/bin/flatpak install -y flathub $app
+      done
+
+      ${pkgs.flatpak}/bin/flatpak uninstall --unused -y
+
+      ${pkgs.flatpak}/bin/flatpak update -y
+    '';
+  };
+}