when selecting a range in the heatmap chart, selection not visible (#1384)

This was a regression from #1347. This adds back the existing styles.

<img width="1210" height="548" alt="Screenshot 2025-11-19 at 9 53 27 AM" src="https://github.com/user-attachments/assets/52343706-2b92-4779-94c0-bdccb1668f09" />

Fixes HDX-2863

Author: brandon-pereira

.changeset/poor-tomatoes-sit.md

@@ -0,0 +1,5 @@
+---
+"@hyperdx/app": patch
+---
+
+Add back selection ui on histogram

packages/app/styles/app.scss

@@ -298,3 +298,9 @@ a,
 [role='button'] {
   cursor: pointer;
 }
+
+.u-select {
+  border: 1px solid #50fa7b;
+  background: #50fa7a0d !important;
+  color: #2E224788;
+}