refactor(nix): use baseline x86 desktop packages

overlays/default.nix

@@ -22,6 +22,10 @@
       system = final.stdenv.hostPlatform.system;
       config.allowUnfree = true;
     };
+
+    # Keep the complete KDE/Qt package scope on the same baseline architecture
+    # so large packages such as Qt WebEngine can use the upstream binary cache.
+    kdePackages = final.x86-v1.kdePackages;
   };
 
   patches = import ./patches;