home: wm: screen-lock: use actual assertion

Instead of hijacking the type verification, use an assertion.

home/wm/default.nix

@@ -65,13 +65,7 @@ in
         enable = my.mkDisableOption "Notify when about to lock the screen";
 
         delay = mkOption {
-          type = with types;
-            addCheck int (x:
-              let
-                cfg = config.my.home.wm.screen-lock.notify;
-                cornerCfg = config.my.home.wm.screen-lock.cornerLock;
-              in
-              (cfg.enable && cornerCfg.enable) -> cornerCfg.delay >= x);
+          type = types.int;
           default = 5;
           example = 15;
           description = ''

home/wm/screen-lock.nix

@@ -12,6 +12,22 @@ let
 in
 {
   config = lib.mkIf cfg.enable {
+    assertions = [
+      {
+        assertion =
+          let
+            inherit (cfg) cornerLock notify;
+            bothEnabled = cornerLock.enable && notify.enable;
+            cornerLockHigherThanNotify = cornerLock.delay >= notify.delay;
+          in
+          bothEnabled -> cornerLockHigherThanNotify;
+        message = ''
+          `config.my.home.wm.notify.delay` cannot have a value higher than
+          `config.my.home.wm.cornerLock.delay`.
+        '';
+      }
+    ];
+
     services.screen-locker = {
       enable = true;