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/ui/windows/mining_tab.h

@@ -15,5 +15,11 @@ namespace ui {
  */
 void RenderMiningTab(App* app);
 
+/**
+ * @brief Returns true when the thread benchmark is actively running.
+ * Used by idle mining to avoid interfering with measurements.
+ */
+bool IsMiningBenchmarkActive();
+
 } // namespace ui
 } // namespace dragonx