fix(frontend): zindex and radius on single selector component (#2276)

# Description

Please include a summary of the changes and the related issue. Please
also include relevant motivation and context.

## Checklist before requesting a review

Please delete options that are not relevant.

- [ ] My code follows the style guidelines of this project
- [ ] I have performed a self-review of my code
- [ ] I have commented hard-to-understand areas
- [ ] I have ideally added tests that prove my fix is effective or that
my feature works
- [ ] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged

## Screenshots (if appropriate):

frontend/lib/components/ui/SingleSelector/SingleSelector.module.scss

@@ -3,6 +3,7 @@
 @use "@/styles/Radius.module.scss";
 @use "@/styles/Spacings.module.scss";
 @use "@/styles/Typography.module.scss";
+@use "@/styles/ZIndexes.module.scss";
 
 .single_selector_wrapper {
   display: flex;
@@ -71,10 +72,11 @@
     top: 100%;
     border: 1px solid Colors.$normal-grey;
     border-top: none;
-    border-radius: 0 0 Radius.$normal Radius.$normal;
+    border-radius: 0 0 Radius.$big Radius.$big;
     overflow: hidden;
     max-height: 180px;
     overflow: scroll;
+    z-index: ZIndexes.$overlay;
 
     .option {
       padding: Spacings.$spacing03;