fix: dropdown buttons obscuring tooltips on the activity calendar (@byseif21) (monkeytypegame#7748)

unable to read the tooltips for the top rows of the activity calendars
under the dropdown button ("last 12 months")

<img width="672" height="314" alt="Screenshot 2026-03-30 205021"
src="https://github.com/user-attachments/assets/04f2e1d9-0e61-47d9-bc30-f16b7317873b"
/>


introduced here
monkeytypegame@2224ed2
not sure what was that fixing?

Author: byseif21

frontend/src/ts/components/ui/SlimSelect.tsx

@@ -482,11 +482,7 @@ export default function SlimSelect(props: SlimSelectProps): JSXElement {
       ref={(el) => (containerRef = el)}
       class="relative [&>.ss-content]:top-full! [&>.ss-content]:left-0! [&>.ss-content]:w-full!"
     >
-      <select
-        ref={(el) => (selectRef = el)}
-        multiple={props.multiple}
-        class="z-10000"
-      >
+      <select ref={(el) => (selectRef = el)} multiple={props.multiple}>
         {props.children}
       </select>
     </div>