Breakout rooms: let the moderator send selected slides to a room

Is your feature request related to a problem? Please describe.

Teachers often present a concept in the main room and then want small groups to work on the next activity in breakout rooms. Today a breakout room can receive either the current slide (one page) or a whole presentation. There is no way to send only the pages the group needs, for example slides 5–7 of a 20-slide lesson.

With young students in particular (feedback from an elementary online school), sending the whole deck means students wander to other slides, and sending only the current slide loses the rest of the activity.

A related discoverability problem, observed on 4.0.0-rc.3: the per-room presentation selector is only rendered when the meeting has more than one presentation (sidebar-create-breakout/sidebar-room-assignment.tsx, presentations.length > 1). With a single presentation the moderator cannot see that rooms will receive the current slide, and cannot choose the whole presentation instead.

Describe the solution you'd like

In the breakout room creation panel, for each room (or for all rooms at once):

• Keep the existing options: "Current slide" and each whole presentation.
• Add "Selected slides": pick a page range or individual pages from a presentation, and send only those pages to the room.
• Always show the selector when there is at least one presentation, so moderators can see and change what the rooms will receive.

Describe alternatives you've considered

• Uploading each practice activity as its own short presentation before class and sending that presentation to the rooms. Works, but needs preparation and extra files.
• Sending the current slide and asking students not to change pages. Loses the multi-page activity.

Additional context

Current behaviour on 4.0.0-rc.3, verified on a test meeting: the default is the current slide of the current presentation; choosing a presentation from the room's selector sends all of its pages. CreateBreakoutRoomsCmdMsg carries presId and allPages per room, so a page range would need a new field (for example pages: [5,6,7]) through akka and bbb-web's breakout presentation copy.