compose: Add a progress indicator for file uploads

[chrisbobbe]

A user reports wanting

#mobile > Change Upload behaviour @ 💬

[…] a bar or even better the percentage of upload, so the user knows that the upload is complete and can send the photo.

It should be possible to listen to the response stream and find percent-completed using content-length.

For the design, I think this Figma frame would work:

https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=4031-8567&m=dev

[chrisbobbe]

It looks like the available tus clients in Dart make it easy to report upload progress; see #959 and https://github.com/jjmutumi/tus_client/blob/master/example/lib/main.dart#L145-L150