fix: resolve CSS linter warning by adding standard mask property

Author: ridash2005

frontend/src/index.css

@@ -255,6 +255,7 @@ input, textarea {
   padding: 1px;
   background: var(--gradient-card-border);
   -webkit-mask: linear-gradient(#fff 0 0) content-box, linear-gradient(#fff 0 0);
+  mask: linear-gradient(#fff 0 0) content-box, linear-gradient(#fff 0 0);
   -webkit-mask-composite: xor;
   mask-composite: exclude;
   opacity: 0;