feat(tab-bar): make box shadow independent of expand mode (#30117)

Issue number: internal
---------

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## What is the current behavior?
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When the tab bar's expand mode is 'full' on the ionic theme, it does not
have a box shadow to separate it from the rest of the page.

## What is the new behavior?
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this PR. -->
- The tab bar on the ionic theme now has a box shadow independently of
its expand mode.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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## Other information

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screenshots of how the component looks before and after the change. -->

core/src/components/tab-bar/tab-bar.ionic.scss

@@ -21,6 +21,8 @@
 
   gap: globals.$ion-space-300;
 
+  box-shadow: #{globals.$ion-elevation-2};
+
   // TODO(ROU-10853): replace this value with a layer token
   z-index: 10;
 }
@@ -54,8 +56,6 @@
 
   width: fit-content;
 
-  box-shadow: #{globals.$ion-elevation-2};
-
   contain: content;
 }