Add the FluffyChat name and version to the userAgent string

[inexcode]

Feature Description

Right now, Fluffy's userAgent is Dart/3.3 (dart:io). It should at least contain something like FluffyChat/1.8.0.

This might be also an issue of the Famedly Matrix SDK if it doesn't expose the userAgent setting and doesn't use a more sensible default value.

Rationale

Using generic library useragents is often discouraged in production environments and is not useful for me as an admin. If the client misbehaves, I should know the exact client and its version.

Mockup

No response

Additional Context

The current userAgent is probably taken from here: https://api.dart.dev/stable/3.3.0/dart-io/HttpClient/userAgent.html

Example of the value suggested in the Matrix chat: FluffyChat/1.8.0 Dart/3.3 Android/14

How it shouldn't look like: element-hq/element-meta#2309

[Mikaela]

Adding keyword user-agent since I had to manually scroll all issues to find this again since GitHub search didn't find either that word or just agent.

[github-actions]

This issue is stale because it has been open for 120 days with no activity.

[Mikaela]

I haven't been shown any server logs recently, nut I think this is still relevant