* fix(frontend): keep the side-chat draft when a send fails
PromptInput clears the composer only when onSubmit resolves and keeps it
on rejection. The side chat's handleSubmit caught every failure, showed
the toast and resolved, so a failed side-chat creation or attachment
upload lost the typed text and attachments. The first message to a new
side chat was also cleared as soon as it was queued, before the queued
effect sent it, so a failure there could not be retried either.
handleSubmit now rethrows after its toast, and the queued first send
carries the submit's resolve/reject so the composer settles with that
send's outcome. The queued effect no longer toasts on its own, so a
failed first send shows one error instead of two.
* docs(changelog): reference #6407
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Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
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