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/app.h

@@ -173,6 +173,9 @@ public:
     // Pool mining (xmrig)
     void startPoolMining(int threads);
     void stopPoolMining();
+    int getXmrigRequestedThreads() const {
+        return xmrig_manager_ ? xmrig_manager_->getRequestedThreads() : 0;
+    }
 
     // Mine-when-idle state query
     bool isIdleMiningActive() const { return idle_mining_active_; }