Messages with image attachments could take a moment to appear (large
base64 upload, server-side resize, first-session open) while the composer
cleared instantly, making it look like nothing happened.
Await the send for attachment messages and surface a "Sending…" button
label plus a "Sending your files…" / "Saving your files…" hint, disabling
the composer until the message lands. Plain text messages stay
fire-and-forget so the input frees up instantly.
