Waterfall Phase 1.2 - Add vertical grid lines for time visualization

waterfall.html

@@ -40,6 +40,15 @@
             transform: translateX(-50%);
         }
         
+        .grid-line {
+            position: absolute;
+            top: 25px;
+            bottom: 0;
+            width: 1px;
+            background: rgba(119, 119, 119, 0.15);
+            pointer-events: none;
+        }
+        
         .waterfall-row {
             display: flex;
             align-items: center;