Add huddle screen sharing controls

[klopez4212]

Summary

• Adds huddle screen sharing over the huddle media path, including relay frame forwarding, desktop capture commands, receiver playout, and the screen-share control surface.
• Reorders the huddle controls to microphone, speaker, screen share, then thread/reactions/more; the overflow menu now holds add agent, transcript, and participants.
• Extends the E2E mock bridge/screenshot helper so huddle controls can be captured in desktop snapshots.

Scope

This PR is intentionally limited to huddle screen sharing and huddle control organization. Spotify and Google Calendar work is not included.

Relay rollout required

Screen sharing only works when the huddle relay is running this PR's relay changes. The desktop client negotiates huddle media protocol v3 with the audio WebSocket; if the relay only supports the older protocol, the client falls back to audio-only and reports screen_share_available=false.

To enable screen sharing in staging or production, ship the updated buzz-relay Docker image through the relay pipeline (sprout-oss image build, then block-coder-tf-stacks/ArgoCD rollout). A desktop-only release cannot enable screen sharing against an old relay.

Screenshots

Huddle controls

Screen share sits after microphone and speaker, before the secondary huddle actions.

Overflow menu

Add agent, transcript, and participants now live under More.

Screen share affordance

The screen-share control has a focused tooltip affordance.

Checks

• cd desktop && pnpm build
• cd desktop && pnpm check
• cargo clippy -p buzz-relay --all-targets -- -D warnings
• just desktop-tauri-check
• Pre-push hooks: rust tests, desktop tests, mobile tests, desktop Tauri tests

Note: just ci currently fails in desktop-tauri-clippy on pre-existing managed-agent clippy warnings in files untouched by this PR (agent_models.rs, config_bridge/reader.rs, managed_agents/runtime.rs, config_bridge/claude.rs).