fix: prevent duplicate workspace chat messages on repeated submit #143
Description
Summary
Submitting a workspace chat message more than once in quick succession can create duplicate messages.
Expected
A repeated click or duplicate POST for the same message submission should result in a single durable message.
Actual
The current message composer can create duplicate workspace_chat_messages rows when the submit action is triggered multiple times before the first response completes.
Notes
This is the message-side counterpart to the chat creation duplicate-submit fix. It should likely add both a submit-once UI guard and a backend idempotency check for POST /chats/{chat}/messages.