fix(mining): make XmrigManager's thread count atomic

The requested thread count is written by generateConfig() on the
mining-control thread and read on the UI thread (the thread-grid sync
under idle thread scaling, the pool auto-balance restart) without any
synchronisation. The write also stored the unclamped count first and
clamped it afterwards, so a reader could see more threads than cores.
It is now an atomic, set by one store of the clamped value.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/daemon/xmrig_manager.h

@@ -173,7 +173,7 @@ private:
     // xmrig HTTP API credentials (random per session)
     int         api_port_  = 0;
     std::string api_token_;
-    int         threads_   = 0;  // Thread count for mining
+    std::atomic<int> threads_{0};  // Thread count for mining (written on the mining-control thread, read by the UI)
     std::string pool_host_;       // Pool hostname for stats API
     PoolStats   stats_;
     mutable std::mutex stats_mutex_;