Add "My threads" Metaspace to Spaces panel

### Your use case

#### What would you like to do?
Will be good to have a "My threads" Metaspace in Spaces panel, like People, Favorites, Other rooms, that will show all threads, in which user is participated, from all available rooms.

#### Why would you like to do it?
At now we have Threads panel in right side, that shows only thread from currently open room. But very often people want to see list of all threads, in which it is participated, from all rooms, like in Slack app.

#### How would you like to achieve it?
Add the "My threads" Metaspace to current Spaces panel. And display unread counter as sum of unread messages from all threads will be awesome!

### Have you considered any alternatives?

Alternatively we can extend right "Threads" panel to show all subscribed spaces (not only from current room), but I think that all right panels now are room-specific, and it's not so good to add some global data to it.

### Additional context

_No response_

Start by tracing the Spaces panel and the room-specific Threads panel, since the issue identifies both as the relevant UI areas. Determine how participated threads are collected across rooms and how unread counts are represented; done means a My threads Metaspace appears in Spaces and shows the requested threads with an aggregate unread count.

Written by the indexing model from the issue text.