Ability to mark all rooms in a space as read at once

### Your use case

#### What would you like to do?
Mark all rooms in a space as 'read' at once, rather than each space individually

#### Why would you like to do it?
This is more convenient than marking each individual item as read, and makes mechanical sense, given rooms are meant to be contained 'within' spaces.

This behavior is common among almost all other chat applications that support multi-room spaces, such as Discord, Revolt, Slack, etc.

#### How would you like to achieve it?
The context menu for the space should have a button as each room does to 'mark as read.'

### Have you considered any alternatives?

Manually marking each room as read is extremely tedious for large spaces. Other alternatives include forcing the user to develop macros to do this, which is not really a solution.

### Additional context

Batch selection for channels already exists when marking them as 'suggested.' This same logic could potentially be used to mark channels as read.

Start by locating the space context menu and the existing room-level “mark as read” action. Review the batch-selection logic used for marking channels as suggested, then verify that the space action marks every room read and that the resulting unread state is reflected consistently.

Written by the indexing model from the issue text.