DropDown buttons fix (tgstation#81808)

## About The Pull Request

Fixes dropdown menu buttons. Postfix increment and decrement were
preventing buttons from working properly, causing menu values to not
change, or to change incorrectly.
## Why It's Good For The Game

It's not just good IT'S AHUENNO AND PIZDATO CUZ IT'S MAKES USING THIS
BEAUTIFUL BUTTONS REAL
<details>
<summary>Screenshot. The left and right buttons I'm talking
about</summary>

![image
1](https://github.com/tgstation/tgstation/assets/114810807/e683b324-ef63-43f3-ad66-1ad9ed518d27)
</details>

## Changelog

🆑
fix: fixed a non-functional buttons of a drop-down menu.
/:cl:

tgui/packages/tgui/components/Dropdown.tsx

@@ -97,9 +97,9 @@ export function Dropdown(props: Props) {
 
       let newIndex = selectedIndex;
       if (direction === 'next') {
-        newIndex = selectedIndex === endIndex ? startIndex : selectedIndex++;
+        newIndex = selectedIndex === endIndex ? startIndex : ++selectedIndex;
       } else {
-        newIndex = selectedIndex === startIndex ? endIndex : selectedIndex--;
+        newIndex = selectedIndex === startIndex ? endIndex : --selectedIndex;
       }
 
       onSelected?.(getOptionValue(options[newIndex]));