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".
Two new methods on `UxFile`, both designed to keep large file content
out of the platform-channel buffer (the failure mode of file_selector
on Android: a ~200 MB video PUT through the Pigeon codec OOM'd the
JVM via `byte[size]` allocation in `FileSelectorApiImpl`).
`UxFile.pick({mimeTypes})` returns `UxPickedFile?` with `path`, `name`,
`mimeType`, `size`. The platform channel reply carries only the
metadata; bytes never cross.
- Android: `ACTION_OPEN_DOCUMENT` + `EXTRA_MIME_TYPES`, registered
as `ActivityResultListener`. On result, stream-copies the SAF
content URI to `cacheDir/ux_pick/<ts>_<safeName>` via an 8 KB
buffer (no full-file allocation in JVM heap), returns the cache
path.
- iOS: `UIDocumentPickerViewController(documentTypes:in: .import)`
— `.import` mode copies the picked file into the app's
Documents/Inbox so the URL is stable. Strong-retained delegate
(the picker's delegate ref is weak).
- macOS: `NSOpenPanel` with `allowedFileTypes`. Sheet-modal when a
Flutter window exists; free-modal otherwise.
`UxFile.videoThumbnail({path, atMs, maxWidth})` returns
`UxVideoThumbnail?` (PNG bytes + dims).
- Android: `MediaMetadataRetriever.getFrameAtTime(..., OPTION_CLOSEST_SYNC)`,
`Bitmap.createScaledBitmap` to maxWidth, PNG-encode via
`ByteArrayOutputStream`, recycle bitmaps in `finally`, release
retriever in `finally`.
- iOS: `AVAssetImageGenerator` with `appliesPreferredTrackTransform = true`,
`maximumSize = (maxWidth, 0)` (preserve aspect), ±500 ms tolerance
for keyframe alignment, decode on `userInitiated` queue.
- macOS: same generator, encoded via `NSBitmapImageRep`.
Compatible with the package's existing iOS 13 / macOS 10.15 deployment
targets — uses legacy `kUTType*` + `UTTypeCreatePreferredIdentifierForTag`
instead of `UTType` (iOS 14 / macOS 11).
