[Backport vscode-v1.52.x] fix(webviews): update z-index for user menu (…

…#6393)

CLOSE
https://linear.app/sourcegraph/issue/CODY-4533/layout-bug-in-prompts-tab-and-new-user-menu-ui


![image](https://github.com/user-attachments/assets/a7d5fe5c-2635-47f1-8ffa-00c781287f62)

Fix UserMenu dropdown appearing underneath PromptList search box
The UserMenu dropdown and PromptList search box were competing for
z-index positioning since both had z-index: 1. Because PromptList was
rendered after UserMenu in the DOM order, it would appear on top of the
UserMenu dropdown when opened.

This fix increases UserMenu's z-index to 2, ensuring it properly
stacks above the PromptList search box while maintaining both
components' ability to overlay their respective underlying content.

Changes

- Updated UserMenu dropdown z-index from 1 to 2 to create proper
stacking hierarchy
- Maintains existing stacking contexts for both components

## Test plan



After


![image](https://github.com/user-attachments/assets/9341c4cf-b90c-4e28-806e-5c6ce15d69ab)

1. Open Cody sidebar
2. Navigate to Prompts tab
3. Click on user avatar to open UserMenu dropdown
4. Verify UserMenu dropdown appears above the PromptList search box
5. Verify PromptList search box still properly overlays its content when
scrolling
6. Verify UserMenu dropdown properly overlays all other UI elements


## Changelog


 <br> Backport 03dbec6 from #6380

Co-authored-by: Beatrix <[email protected]>

vscode/webviews/tabs/TabsBar.module.css

@@ -11,7 +11,7 @@
     container-type: inline-size;
     container-name: tabs-container;
     isolation: isolate;
-    z-index: 1;
+    z-index: 2;
 }
 
 .tabs-container {