Screen sharing and stage-channel controls

## Summary
Implement screen-sharing controls and stage-style channel controls once the voice foundation exists.

## Scope
- Define product behavior for screen sharing within voice sessions.
- Define stage-style audience/speaker/moderator controls adapted to a single guild.
- Support moderation actions around speaking permissions and stage handoffs.
- Keep web-desktop-first behavior coherent before broader client parity.

## Acceptance Criteria
- [ ] Stage-style roles and controls are explicit and permission-aware.
- [ ] Screen-share entry points and moderation controls are understandable in the UI.
- [ ] Audience/speaker transitions are visible and recover correctly after reconnects.
- [ ] The feature boundaries with core voice transport and permissions are clear.

## Operational Notes
- This issue depends on voice-channel signaling and permission work being trustworthy first.
- Self-hosted operators need explicit controls to disable or limit costly realtime features.