Deliver the DOM paste event as a full ClipboardItem (text and image
files) through a new InputHandler::paste hook instead of inserting only
plain text, and add Platform::read_from_clipboard_async so app-initiated
paste (e.g. a custom context menu) can use the browser's asynchronous,
permission-gated clipboard API. Desktop platforms are unaffected: the
async read defaults to a ready task wrapping the synchronous read, and
the paste hook defaults to plain-text insertion.

Release Notes:
- N/A