feat(call): support being in a call from multiple tabs/devices

[HexaField]

Summary

Lets the same agent be in a call from multiple tabs/devices at once. Previously a second tab joining a call hit You are already in a call in another tab. — the call + presence stack was keyed by DID, so two sessions of one agent collided on a single presence entry and a single peer-connection slot.

This changes the unit of call identity from DID to a session key (did::sessionId, reusing the per-tab UUID already in sessionStorage) and decouples calls from tab-coordinator leadership.

Note: multi-device with different DIDs already worked (independent coordinators). This is what makes the same agent work across tabs/devices.

Participant model

Per the agreed design: show every video/screenshare feed separately, dedupe non-video feeds — each person has one representation but all their media is shown.

• Every session with a camera/screenshare feed → its own tile
• Sessions with no video collapse to a single avatar tile per person
• Audio is deduped to one feed per person (no echo)

Changes

• types — AgentState gains did + sessionId
• useTabCoordinator — remove call-pinning (call-pinned/otherTabInCall/setInCall); leadership now only dedupes general presence
• useSignallingService — presence keyed by session; broadcast when leader || inCall; getAgentsInChannel/getAgentsInCall dedupe to one entry per person
• webrtcStore — session-keyed peer connections, session-precise WebRTC signalling, connects to your own other sessions, removes the error toast
• call UI (useVideoLayout/VideoGrid/MediaPlayer) — per-session tiles (did:sessionId:streamKind) + audio-dedupe mute path
• new utils/callSessions.ts — pure helpers (sessionKey, shouldInitiate, dedupeByDid, deriveRemoteSessionTiles) with unit tests

Edge case (handled)

If a person's only mic-on session has its camera off while another of their sessions shows video, an extra avatar tile is emitted so their audio is never silently dropped (see comment in deriveRemoteSessionTiles).

Testing

• ✅ 10 new unit tests for callSessions pass
• ✅ vue-tsc clean for all touched files
• ⚠️ Full app run / two-tab + second-device smoke test still needs a real executor (ad4m-flux-launch.sh) — not run in this environment

🤖 Generated with Claude Code

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• Commit unit tests in branch feat/multi-session-calls

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