Allow chat messages with only an attachment

[hamzamerzic]

Summary

• allow completed image and file attachments to be sent without accompanying text
• preserve attachment-only messages through queued Stop/re-send and Send-now flows
• validate the payload before keyboard, scroll, voice, or queue-layout side effects
• cover the real upload-to-empty-message backend contract

Testing

• npm test (1,864 tests)
• npm run build
• pytest tests/test_chats.py tests/test_uploads.py -q (35 passed)
• rendered browser check: an empty textarea with a completed image exposes an enabled Send action