fix(tab-group): ensure indicator is visible with 1px track-width

The indicator was invisible when --track-width was set to 1px because
box-sizing: border-box (inherited from component styles) caused the
border to be drawn inside the element's 0-height content area. Adding
box-sizing: content-box to the indicator ensures the border extends
outside the content area and remains visible.

Author: root

.branch_name

@@ -0,0 +1 @@
+fix/issue-2415-tab-indicator-track-width

src/components/tab-group/tab-group.styles.ts

@@ -21,6 +21,7 @@ export default css`
 
   .tab-group__indicator {
     position: absolute;
+    box-sizing: content-box;
     transition:
       var(--sl-transition-fast) translate ease,
       var(--sl-transition-fast) width ease;