feat: thread benchmark, GPU-aware idle mining, thread scaling fix

- Add pool mining thread benchmark: cycles through thread counts with
  20s warmup + 10s measurement to find optimal setting for CPU
- Add GPU-aware idle detection: GPU utilization >= 10% (video, games)
  treats system as active; toggle in mining tab header (default: on)
  Supports AMD sysfs, NVIDIA nvidia-smi, Intel freq ratio; -1 on macOS
- Fix idle thread scaling: use getRequestedThreads() for immediate
  thread count instead of xmrig API threads_active which lags on restart
- Apply active thread count on initial mining start when user is active
- Skip idle mining adjustments while benchmark is running
- Disable thread grid drag-to-select during benchmark
- Add idle_gpu_aware setting with JSON persistence (default: true)
- Add 7 i18n English strings for benchmark and GPU-aware tooltips

src/daemon/xmrig_manager.h

@@ -88,6 +88,10 @@ public:
     const PoolStats& getStats() const { return stats_; }
     const std::string& getLastError() const { return last_error_; }
 
+    /// Thread count requested at start() — available immediately, unlike
+    /// PoolStats::threads_active which requires an API response.
+    int getRequestedThreads() const { return threads_; }
+
     /**
      * @brief Get last N lines of xmrig stdout (thread-safe snapshot).
      */