fix: merge services.displayManager into single block

NixOS doesn't allow duplicate attribute set definitions in the
same module. Combine sessionPackages and autologin config.

roles/plasma-bigscreen/default.nix

@@ -77,7 +77,13 @@ in
     services.desktopManager.plasma6.enable = true;
 
     # Register the bigscreen session with the display manager
-    services.displayManager.sessionPackages = [ plasma-bigscreen ];
+    services.displayManager = {
+      sessionPackages = [ plasma-bigscreen ];
+    } // optionalAttrs cfg.autologin {
+      autoLogin.enable = true;
+      autoLogin.user = cfg.user;
+      defaultSession = "plasma-bigscreen-wayland";
+    };
     xdg.portal.configPackages = [ plasma-bigscreen ];
 
     # Fix homescreen not being focused after quitting app or on boot
@@ -123,10 +129,5 @@ in
       };
     };
 
-    services.displayManager = mkIf cfg.autologin {
-      autoLogin.enable = true;
-      autoLogin.user = cfg.user;
-      defaultSession = "plasma-bigscreen-wayland";
-    };
   };
 }