Flutter's Clipboard API only exposes text shapes. Banlu's chat composer
needs image bytes from the system clipboard for desktop paste, so add
a UxClipboard facade backed by per-platform native plugins:
* iOS: prefer raw PNG/JPEG bytes off the pasteboard, fall back to
re-encoding `UIPasteboard.image` as PNG.
* macOS: prefer NSPasteboard `.png`, fall back to TIFF transcoded
through NSBitmapImageRep so screenshots / Preview hand-offs still
work.
* Android: read primary ClipData's first item URI and stream the bytes
through ContentResolver — don't trust the clip description's MIME,
copy whatever the resolver returns.
Returns null (never throws) when the clipboard has no image — callers
treat null as "fall through to text paste".
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