feat: declaratively install flatpak

modules/linux/flatpak/default.nix

@@ -0,0 +1,45 @@
+# NOTE: this module is IMPURE. Flatpaks are declaratively specified but not
+# versioned. Therefore, they are not included in generational rollbacks and
+# persist between generations. This is not ideal, but at least it is a better
+# situation than imperative installation
+{ inputs, ... }:
+{
+  imports = [
+    inputs.nix-flatpak.nixosModules.nix-flatpak
+  ];
+
+  services.flatpak = {
+    enable = true;
+
+    overrides = {
+      global = {
+        Context.sockets = [
+          "wayland"
+          "!x11"
+          "!fallback-x11"
+        ];
+
+        Environment = {
+          # Fix un-themed cursor in some Wayland apps
+          XCURSOR_PATH = "/run/host/user-share/icons:/run/host/share/icons";
+        };
+      };
+    };
+
+    update.auto = {
+      enable = true;
+      onCalendar = "weekly";
+    };
+
+    packages = [
+      {
+        flatpakref = "https://sober.vinegarhq.org/sober.flatpakref";
+        sha256 = "sha256:1pj8y1xhiwgbnhrr3yr3ybpfis9slrl73i0b1lc9q89vhip6ym2l";
+      }
+      {
+        appId = "org.vinegarhq.Sober";
+        origin = "sober";
+      }
+    ];
+  };
+}