Allow renaming threads in Element Desktop

[Mushy-Snugglebites-badonkadonk]

What would you like to do?

I’d like Element Desktop to support renaming threads.

Why would you like to do it?

When using Matrix threads to multitask with AI agents, a thread name would make it much easier to tell at a glance what each thread is for. Right now, active threads can become hard to distinguish once there are several concurrent agent conversations.

How would you like to achieve it?

Add a way to rename a thread from the thread UI, with the title shown in the thread list and thread header.

Additional context

This is especially useful for the official Element Desktop app on macOS, where threads can act as a lightweight way to manage multiple parallel agent conversations.

Related issue: element-hq/element-web#33769