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/util/platform.h

@@ -131,6 +131,14 @@ public:
      * @return Seconds since last user input, or 0 on failure
      */
     static int getSystemIdleSeconds();
+
+    /**
+     * @brief Get GPU utilization percentage (0–100).
+     * Linux: reads sysfs for AMD, /proc for NVIDIA.
+     * Windows: queries PDH GPU engine counters.
+     * @return GPU busy percent, or -1 if unavailable.
+     */
+    static int getGpuUtilization();
 };
 
 /**