fix(rpc): abort in-flight curl on disconnect/shutdown to avoid UI freezes

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>

src/app_network.cpp

@@ -507,7 +507,9 @@ void App::onDisconnected(const std::string& reason)
     address_validation_cache_dirty_ = true;
     resetTransactionHistoryCacheSession();
 
-    // Tear down the fast-lane connection
+    // Tear down the fast-lane connection. Signal abort first so a fast-lane call blocked in
+    // curl_easy_perform unblocks and stop()'s join() returns promptly (no UI freeze).
+    if (fast_rpc_) fast_rpc_->requestAbort();
     if (fast_worker_) {
         fast_worker_->stop();
     }