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/rpc/rpc_client.h

@@ -5,6 +5,7 @@
 #pragma once
 
 #include "types.h"
+#include <atomic>
 #include <string>
 #include <functional>
 #include <memory>
@@ -87,6 +88,18 @@ public:
      */
     bool isConnected() const { return connected_; }
 
+    /**
+     * @brief Ask an in-flight call() to abort as soon as possible (thread-safe).
+     *
+     * Set from another thread (e.g. before stop()-ing the worker on disconnect/shutdown):
+     * a curl progress callback aborts the transfer, so a blocked curl_easy_perform returns
+     * promptly instead of freezing the UI thread's join() until the request timeout. Cleared
+     * on the next connect(); abortRequested() is read by the progress callback.
+     */
+    void requestAbort();
+    void clearAbort();
+    bool abortRequested() const noexcept { return abort_.load(std::memory_order_relaxed); }
+
     /**
      * @brief True if the last connect() succeeded but daemon returned a warmup error.
      * The curl handle is valid and auth succeeded — RPC calls will throw warmup errors
@@ -236,6 +249,7 @@ private:
     std::string port_;
     std::string auth_;  // Base64 encoded "user:password"
     bool connected_ = false;
+    std::atomic<bool> abort_{false};  // set cross-thread to abort an in-flight transfer
     bool warming_up_ = false;
     std::string warmup_status_;
     std::string last_connect_error_;