[UEPR-546] Use configurable assetHost for library asset thumbnails

[adzhindzhi]

Resolves

UEPR-546

Proposed Changes

• Removе the hardcoded assetServiceUri field from the objects returned by getItemIcons() and from ScratchImage props
• Remove the platform check that rendered a plain <img src={assetServiceUri}> on web. All platforms now go through <ScratchImage>, with the platform prop passed through so ScratchImage can adjust its behaviour.
• Add platform-based parallelism for ScratchImage: web sets _maxParallelism = Infinity (no cap), while desktop/Android keeps 6 (loading up to 6 parallel items on web adds a significant visual delay compared to previous behavior)
• Replace the VisibilitySensor render-prop pattern, which called loadPendingImages() on every render, with the onChange callback, which fires only when visibility actually changes.
• Add a static _assetCache Map so assets already fetched in a session are served from memory on subsequent requests.
• Маke sure icon rotation works as expected with the updated logic

Reason for Changes

Library thumbnail URLs were hardcoded to https://cdn.assets.scratch.mit.edu, making the asset host non-configurable and bypassing the scratch-storage layer entirely. This meant the assetHost prop passed to the GUI had no effect on library thumbnails, making other platforms unable use a different asset host. Routing everything through scratch-storage's registered web stores makes the host fully configurable via the standard assetHost prop.