Support drag-and-drop image uploads in Chat

[angnuoli]

Background

Chat currently does not support adding local images by dragging them into the interface. Users should be able to drop image files into the Chat input area and send them as message attachments.

Requirements

• Support dragging one or more images into the Chat input area.
• Validate supported image formats and file size limits.
• Show thumbnails before sending and allow users to remove selected images.
• Integrate image attachments with the existing message sending flow so the model can receive and interpret them.
• Show clear feedback for unsupported files, oversized files, and send failures.

Acceptance criteria

• Users can add and send images by dragging them into Chat.
• Multiple images can be previewed, removed, and selected again as expected.
• Invalid files are not sent, and the UI displays an understandable error message.