desktop-jovian: unify steam/jovian config across yarn + patiodeck

- modules/desktop-jovian.nix: shared Jovian deck-mode config (unfree
  predicate, jovian.steam, sddm, gamescope override, imports
  desktop-steam-update.nix)
- home/progs/steam-shortcuts.nix: declarative non-Steam shortcuts
  (Prism Launcher); add new entries here for all Jovian hosts
- hosts/yarn/default.nix: reduced to host-specific config only
- hosts/patiodeck/default.nix: same

modules/desktop-jovian.nix

@@ -0,0 +1,40 @@
+# Jovian-NixOS deck-mode configuration shared by all hosts running Steam
+# in gamescope (yarn, patiodeck). Host-specific settings (like
+# jovian.devices.steamdeck.enable) stay in the host's default.nix.
+{
+  lib,
+  username,
+  inputs,
+  ...
+}:
+{
+  imports = [
+    ./desktop-steam-update.nix
+    inputs.jovian-nixos.nixosModules.default
+  ];
+
+  nixpkgs.config.allowUnfreePredicate =
+    pkg:
+    builtins.elem (lib.getName pkg) [
+      "steamdeck-hw-theme"
+      "steam-jupiter-unwrapped"
+      "steam"
+      "steam-original"
+      "steam-unwrapped"
+      "steam-run"
+    ];
+
+  jovian.steam = {
+    enable = true;
+    autoStart = true;
+    desktopSession = "niri";
+    user = username;
+  };
+
+  # jovian overrides the display manager; sddm is required
+  services.displayManager.sddm.wayland.enable = true;
+
+  # desktop-common.nix enables programs.gamescope which conflicts with
+  # jovian's own gamescope wrapper
+  programs.gamescope.enable = lib.mkForce false;
+}