desktop: allow paste-to-post for images in galleries

[patosullivan]

fixes tlon-3863

At first, I attempted to implement this in app/ui/components/Channel/index.tsx, but I ended up implementing this in ChannelScreen using useUploadState directly and not using the attachment context because:

1) It avoids race conditions between attachment state updates and upload operations (calling attachAssets and waitForAttachmentUploads from the attachment context in succession was fraught).
2) It works within component hierarchy constraints (ChannelScreen is a parent of AttachmentProvider and it feels like this functionality should be in ChannelScreen).
3) It provides granular control for auto-posting each image as a separate message immediately after upload completion

Open to ideas if anyone can think of a better approach, though.

I added a new method to our attachment context that handles this case.

[linear]

TLON-3863 Paste to post to gallery

[dnbrwstr]

Would it be possible to extend or modify the attachment context to work better here? This seems like something it really should be able to handle.

[patosullivan]

Would it be possible to extend or modify the attachment context to work better here? This seems like something it really should be able to handle.

@dnbrwstr maybe we could add a new method to AttachmentState? uploadAndAutoPost or similar.

[patosullivan]

@dnbrwstr updated to use the context