162 add loader color and background customization

[marceloams]

Add loader color and background customization

Adds optional props to customize loader appearance with improved layering.

Changes

• Added loaderColor and loaderBackgroundColor props
• Hide loader with opacity animation when content loads
• Fixed z-index layering for proper visibility

Usage

<InstagramStories 
  stories={stories}
  loaderColor="#FF0000"
  loaderBackgroundColor="rgba(0,0,0,0.5)"
/>

Note: I've noticed that the loader wasn't being displayed properly above content. Since the hiding state is now fixed, I've also updated the z-index values to show it properly above the content, while keeping the progress header showing as well.

Closes #162

[marceloams]

Hey @Lipo11, I found a zIndex typo and noticed it should be in List.styles.ts container instead of Progress.styles.ts. Could I create a PR to fix this or open an issue first?

[Lipo11]

feel free to create a PR :) thanks