feat(collab): broadcast user message and AI stream to relay

[andrewchumchal]

Problem

When a user sends a message in a live collaboration room, the message and the AI response stream were never forwarded to the relay server. Other participants couldn't see any messages or the AI response.

Root causes

1. ipc.ts CHAT_SEND handler streamed AI events to the renderer via wc.send(...) but never called sendToRoom(...) to broadcast to the relay.
2. The relay's assertLock rejected all sends when lockHolder === null (nobody has explicitly requested a turn), blocking the host from broadcasting their first message.
3. The relay's handleMessageAdd used broadcastAll (echoed back to sender), which would cause duplicate messages for the sender.

Changes

packages/desktop/src/main/ipc.ts

• Before the iteration loop: broadcast the user message via message_add and signal stream_start
• Inside the delta handler: forward each chunk as stream_chunk
• On completion (no tool calls): broadcast stream_end with the final assistant message

sendToRoom is a no-op when no collaboration session is active, so these calls are safe in all normal (non-collab) conversations.

Relay server (separate PR / already pushed to relay-server main)

• assertLock: allow sending when lockHolder === null (no one has claimed a turn yet)
• handleMessageAdd: switched from broadcastAll → broadcastExcept to prevent the sender receiving their own message back as a duplicate