The backend uses client_id to identify the sender and exclude them
from receiving their own message via WebSocket (avoiding duplication
with optimistic update).
Changes:
- rest.js: Add clientId parameter to sendChatMessage
- sessionChatSlice.js: Pass clientId to API call
- JKChatComposer.js: Get server.clientId from context and pass it
This fixes: Messages now broadcast to OTHER users in session, not
just the sender. The sender sees optimistic update immediately, other
users receive via WebSocket.
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
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