Notification Decoration Revised

- Add feature flag `RoomListActivityVisibility` with 3 states:
- `current`: show unread badges for all unread messages (the default).
- `show`: don't show badges but bold unread room names and their latest message
- `hide`: don't show badges and don't bold room names and messages

- Remove the grey dots for when there is activity in rooms set to mentions or mute.
- For all unread rooms, make the message preview text bold for better legibility
- For rooms with no unreads, make the font weight regular, again for legibility

iOS PR: https://github.com/element-hq/element-x-ios/pull/5312
Design: https://www.figma.com/design/aC1IwPRyPQcAXHtMF1L0MR/Hackathon-2026?node-id=603-28191