fix: input validation, null guard and code comments in app.js/style.css

style.css

@@ -40,6 +40,7 @@ main { padding: 1.5rem; max-width: 900px; margin: 0 auto; }
 .tab-content { display: none; }
 .tab-content.active { display: block; }
 
+/* --- Shared component classes (used across tabs) --- */
 .color-preview {
   width: 100%;
   height: 80px;