Add support for custom selected text in Dropdown component

[narender56]

Description

• Added a new prop selectedText to let users set their own text to show for the selected value in the dropdown. By default, it shows the selected item label, but now you can override it with any string you want.

Why?

• Original dropdown didn’t let me customize the displayed selected text, which I needed for my use case. This adds that flexibility without breaking anything.

How to use:

<Dropdown
  data={data}
  value={selectedValue}
  selectedText={"You picked: " + selectedValue}
  ...
/>