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/config/settings.h

@@ -221,6 +221,8 @@ public:
     void setIdleThreadsActive(int v) { idle_threads_active_ = std::max(0, v); }
     int getIdleThreadsIdle() const { return idle_threads_idle_; }
     void setIdleThreadsIdle(int v) { idle_threads_idle_ = std::max(0, v); }
+    bool getIdleGpuAware() const { return idle_gpu_aware_; }
+    void setIdleGpuAware(bool v) { idle_gpu_aware_ = v; }
 
     // Saved pool URLs (user-managed favorites dropdown)
     const std::vector<std::string>& getSavedPoolUrls() const { return saved_pool_urls_; }
@@ -317,6 +319,7 @@ private:
     bool        idle_thread_scaling_ = false; // scale threads instead of start/stop
     int         idle_threads_active_ = 0;    // threads when user active (0 = auto)
     int         idle_threads_idle_   = 0;    // threads when idle (0 = auto = all)
+    bool        idle_gpu_aware_  = true;     // treat GPU activity as non-idle
     std::vector<std::string> saved_pool_urls_; // user-saved pool URL favorites
     std::vector<std::string> saved_pool_workers_; // user-saved worker address favorites