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feat(playground): rewind-and-retry when editing the streaming message

When the user clicks Edit on the last user message while the assistant
is currently streaming a response, the submit button switches to a
Resend action. Clicking it cancels the active stream, drops the partial
assistant response and the original user message, and sends the new
text as a fresh message — replacing the in-flight turn with the edited
one. A small chip above the composer surfaces the mode and offers a
cancel.

Persists the trimmed messages via the existing
`chat:update-thread-messages` IPC handler so a navigation mid-stop
can't resurrect the partial state.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>