refactor(home): default hyprland kb_layout to neutral us

the baseline hard-coded a german layout, leaking a personal value into a
module meant to be region-neutral. ship `mkDefault "us"` like `console.keyMap`
so a consumer overrides it from their inventory.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

modules/home.nix

@@ -595,7 +595,8 @@ in
       };
 
       input = {
-        kb_layout = "de";
+        # Neutral default; set the real layout in your inventory (like console.keyMap).
+        kb_layout = lib.mkDefault "us";
         follow_mouse = 1;
         numlock_by_default = true;
       };