stop()-ing a worker that is mid curl_easy_perform joined on the UI thread, so a
slow/hung transfer froze the UI until the request timeout. Add RPCClient::
requestAbort() (a thread-safe atomic read by a curl progress callback that aborts
the transfer), and call it before stopping the workers on disconnect
(onDisconnected) and shutdown (beginShutdown + the synchronous fallback). The
flag is cleared on each connect() so a fresh connection never starts aborted.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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