Room list: mentions in low-priority rooms are not low priority for me

### Steps to reproduce

1. Mark a room as low priority
2. Someone mentions you in that room
3. The room shows a green icon next to it, but remains in the Low Priority section, so I can't see it because it's scrolled off the bottom

### Outcome

#### What did you expect?

If someone mentions me, even in a low-priority room, I want to be able to see it immediately.

Similarly, if someone invites me or mentions me, but I currently have the Favourites section expanded, I don't see it, because it appears in the Chats section.

I know I have banged on about it before, but I would like a flat list of rooms (no sections) where the most important things appear at the top. In this case, a low-priority room with a mention would be more important than e.g. an unread normal room.

#### What happened instead?

I didn't notice I was mentioned because it was scrolled off the bottom of the room list.

### Application version

Element version: 78273c569b26-js-171029678727 Crypto version: Rust SDK 0.18.0 (6fe03c1), Vodozemac 0.10.0

No files or tests are named. Start by reproducing the room-list behavior in Element Web with a low-priority room, a mention, and expanded Favourites; trace how rooms are grouped and ordered. Done means mentions and invitations are surfaced where the user can see them immediately, including from low-priority rooms, with the expected ordering agreed and covered by tests.

Written by the indexing model from the issue text.