ReadText returns the clipboard's text content, or "" if the clipboard holds
no text. The text is returned verbatim — no trimming — so a round-trip
through WriteText preserves leading/trailing whitespace.
Provider is a native clipboard backend. On mobile the shell registers one
via SetProvider (ClipboardManager on Android, UIPasteboard on iOS); on
desktop the built-in platform implementation is used when no provider is
set.