perf(app): run miner shutdown off the UI thread

At quit the UI thread drained and joined the mining-control thread and
then called the miner's blocking stop with a 3 s budget. Both ran on the
UI thread, so the window could stall for up to that long before the
shutdown screen rendered.

- Quit now only signals the mining-control thread (no join) on the UI
  thread. The blocking teardown — join the control thread, then stop the
  miner — moves to a background thread, so xmrig still has a single
  driver but the UI thread never blocks on it.
- On the daemon-stop path that teardown runs first on the shutdown
  thread, so the miner is fully stopped before the daemon stop begins.
  On the disconnect-only quit paths it runs on its own thread, which
  the final shutdown joins before tearing xmrig down, so there is never
  a second concurrent join of the control thread.

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

src/app.h

@@ -1196,7 +1196,9 @@ private:
     std::deque<std::function<void()>> mining_ctl_queue_;
     bool mining_ctl_stop_ = false;
     void postMiningControl(std::function<void()> job);  // enqueue a blocking xmrig op onto the FIFO thread
+    void signalMiningControlStop();                     // signal only (no join): safe on the UI thread
     void stopMiningControlThread();                     // signal + join the control thread (shutdown)
+    void finishMinerShutdown();                         // join control thread -> xmrig stop(3000); background only
     // Builds the pool config on the main thread and posts stop(if running)+start to the control thread.
     // retargetOnly: a thread-count change, decided on the control thread (never starts a miner).
     void postPoolMinerJob(int threads, bool retargetOnly);
@@ -1262,6 +1264,7 @@ private:
     bool address_list_dirty_ = false;   // P8: dedup rebuildAddressList
     GuardedStatus shutdown_status_;   // thread-safe: written by the shutdown thread, read by the UI (M-05)
     std::thread shutdown_thread_;
+    std::thread miner_stop_thread_;   // disconnect-only quit paths: miner teardown off the UI thread
     // Confirm-before-stopping-daemon-mid-witness-rebuild guard (see beginShutdown / renderDaemonStopConfirm)
     bool pending_shutdown_confirm_ = false;       // a quit is deferred, waiting to open the confirm modal
     bool daemon_stop_confirm_open_ = false;       // the confirm modal is currently showing