fix(rpc): keep the worker alive on non-std throws; atomic flags

The RPC worker loop only caught std::exception. A task that threw
anything else - a non-std exception propagating out of a posted job -
escaped the loop and tore down the worker thread, so every later RPC
silently went unanswered.

- The worker loop now also has a catch-all that swallows non-std
  throws and keeps the thread running. Such a job still loses its
  main-thread callback, exactly as the std::exception path already
  did; the only change is that the worker survives.
- RPCClient's connected_ and warming_up_ flags are now atomic. They
  are read from the UI thread while the worker updates them, so the
  plain bools were a data race.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/rpc/rpc_client.h

@@ -301,9 +301,9 @@ private:
     std::string host_;
     std::string port_;
     std::string auth_;  // Base64 encoded "user:password"
-    bool connected_ = false;
+    std::atomic<bool> connected_{false};
     std::atomic<bool> abort_{false};  // set cross-thread to abort an in-flight transfer
-    bool warming_up_ = false;
+    std::atomic<bool> warming_up_{false};
     std::string warmup_status_;
     std::string last_connect_error_;
     RpcFailureKind last_connect_kind_ = RpcFailureKind::None;