Update wallpaper to SoTtE and tweak greetd command

Replace the commented Avatar wallpaper with SoTtE-1.jpg as the active
background for all hosts.
Add SoTtE-2.jpg as an alternative wallpaper.
Change greetd's tuigreet command to invoke start-hyprland.

modules/core/greetd.nix

@@ -8,7 +8,7 @@
     #vt = 3;
     settings.default_session = {
       user = username;
-      command = "${pkgs.tuigreet}/bin/tuigreet --time --cmd Hyprland"; # start Hyprland with a TUI login manager
+      command = "${pkgs.tuigreet}/bin/tuigreet --time --cmd start-hyprland"; # start Hyprland with a TUI login manager
     };
   };
 }