Allow sending chat requests with attachments only

[federicobrancasi]

When using chat, attaching an image or file currently requires typing text before the request can be sent.

This makes attachment-only prompts awkward, especially for screenshots or files where the attachment itself is the complete request context.

Expected behavior:

• Users can send a chat request with only explicit attachments, such as images, files, etc.
• The UI should not render an empty-looking request.
• For attachment-only requests, the user-visible request text should be a localized summary such as:
  • Attached 1 image
  • Attached 2 images
  • Attached 1 file
  • Attached 2 files
• Or find a better way to do that

Attached photo: attachment selected, but send is blocked until text is typed.