Current room is not visible in room list with "Mentions" filter

### Steps to reproduce

1. Enable the "Mentions" filter.
2. Open a room.
3. Room disappears from room list (it no longer has a mention).
4. Unable to access context menu for room (eg, "mark as unread", change notification settings, copy room link).

I wonder if the current room should be special-cased so that it always appears in the sidebar, even if it would be excluded by the filters.

### Outcome

#### What did you expect?
The room to remain until the user moved to another room/filter.
#### What happened instead?
It immediately vacated the filter.

### Operating system

Linux Mint

### Application version

Element Nightly version: 2026082501

### How did you install the app?

element.io/get-started

### Homeserver

matrix.org

### Will you send logs?

No

No file or test is named in the issue. Start by locating the room-list filtering entry point and the handling of the active room when the "Mentions" filter is enabled. Reproduce the behavior, then verify that the current room remains available for its context-menu actions until the user changes rooms or filters.

Written by the indexing model from the issue text.