Enable system flatpak via nix-flatpak and simplify HM

modules/system/flatpak.nix

@@ -0,0 +1,25 @@
+{ lib, ... }:
+
+{
+  services.flatpak = {
+    enable = true;
+    remotes = [
+      {
+        name = "flathub";
+        location = "https://dl.flathub.org/repo/flathub.flatpakrepo";
+      }
+      {
+        name = "flathub-beta";
+        location = "https://dl.flathub.org/beta-repo/flathub-beta.flatpakrepo";
+      }
+    ];
+    update.auto = {
+      enable = true;
+      onCalendar = "daily";
+    };
+    packages = [
+      "com.github.tchx84.Flatseal"
+      "io.github.kolunmi.Bazaar"
+    ];
+  };
+}