feat: add support for flatpak in NixOS configuration

Added the necessary configurations and dependencies to enable Flatpak in the Fuchsia module.
This includes enabling the xdg portal for default settings and adding the flatpak service.

nixos/common/optional/persist.nix

@@ -10,6 +10,7 @@
       "/var/log"
       "/var/lib/bluetooth"
       "/var/lib/docker"
+      "/var/lib/flatpak"
       "/var/lib/nixos"
       "/var/lib/systemd/coredump"
       "/etc/NetworkManager/system-connections"

nixos/fuchsia/configuration.nix

@@ -12,6 +12,7 @@
 
     # Services
     ./services/amdgpu-clocks
+    ./services/flatpak
     ./services/libinput
     ./services/ollama
     ./services/printers

nixos/fuchsia/services/flatpak/default.nix

@@ -0,0 +1,10 @@
+{pkgs, ...}: {
+  # Required to install flatpak
+  xdg.portal = {
+    enable = true;
+    config.common.default = ["gtk"];
+    extraPortals = [pkgs.xdg-desktop-portal-wlr];
+  };
+
+  services.flatpak.enable = true;
+}