Support pasting clipboard screenshots into Chat

[angnuoli]

Background

Users should be able to paste images copied from the system screenshot tool or another application directly into Chat without first saving them as local files.

Requirements

• Listen for paste events in the Chat input area and detect clipboard images.
• Convert screenshots into the shared image attachment representation, reusing the preview, removal, validation, and sending flow used by drag-and-drop uploads.
• Preserve sensible behavior when the clipboard contains both text and images, without affecting normal text pasting.
• Support common screenshot formats and show clear feedback when reading or sending an image fails.

Acceptance criteria

• After copying a screenshot, users can add it to Chat with Ctrl+V.
• The pasted image is previewed and can be removed or sent with a message.
• Normal text pasting continues to work as expected.