Change "Favourites" to "Pinned" And Make a "Favourites" Filter

### Your use case

#### What would you like to do?
- Change the "Favourites" section title to "Pinned".
- Make a "Favourites" filter with the associated functions/features.
- Give "Favourites" priority in the room list.

E.g.
1. When you receive a message from a favourited chat, the chat would remain at the top of the list until read.
After being read, more recent activity may move to the top, over it.
#### Why would you like to do it?
It frees up space for users who don't also want their favourites, pinned.

#### How would you like to achieve it?
Change the current "Favourites" section title to "Pinned"
And make "Favourites" into a filter, giving it priority in the room list.
E.g When you receive a message from a favourited chat, the chat would remain at the top of the list until read.

### Have you considered any alternatives?

_No response_

### Additional context

> And "Favorites" should not be a section, as it is now - moving a favorite to a section unfavs it, and favving a room removes it from the section it is in.
https://github.com/element-hq/element-web/issues/33727#issuecomment-4937891574

Start by reviewing the current Favourites section and room-list behavior in Element Web, then read the linked discussion in issue 33727 for related context. Done means Favourites is a filter rather than a section, the section is renamed Pinned, and favourited rooms receive the requested unread-priority behavior.

Written by the indexing model from the issue text.