In user-agent, include platform, platform version, and app version

[chrisbobbe]

In the first pass for

• Set the HTTP user agent #406

, Greg suggests leaving out the platform, platform version, and app version. So I expect the user agent will look something like

ZulipMobile (actually ZulipFlutter)

instead of e.g.

ZulipMobile (Android 14) (actually ZulipFlutter/1.2.3)

. This issue is for filling in those missing things.

[gnprice]

In particular, after #471 there will be (as discussed at #471 (comment)) a sort of residual version of the following issue:

• Login notifications say "An unknown browser on an unknown operating system" #460

which is that the login emails will say "Device: Zulip on an unknown operating system" rather than the actual operating system.

Fixing this issue should fix that. When we have a PR for this issue, one thing we should manually test in it is that it does indeed fix that.