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.cpp

@@ -153,6 +153,7 @@ bool Settings::load(const std::string& path)
         if (j.contains("idle_thread_scaling"))  idle_thread_scaling_ = j["idle_thread_scaling"].get<bool>();
         if (j.contains("idle_threads_active")) idle_threads_active_ = j["idle_threads_active"].get<int>();
         if (j.contains("idle_threads_idle"))   idle_threads_idle_   = j["idle_threads_idle"].get<int>();
+        if (j.contains("idle_gpu_aware"))       idle_gpu_aware_      = j["idle_gpu_aware"].get<bool>();
         if (j.contains("saved_pool_urls") && j["saved_pool_urls"].is_array()) {
             saved_pool_urls_.clear();
             for (const auto& u : j["saved_pool_urls"])
@@ -250,6 +251,7 @@ bool Settings::save(const std::string& path)
     j["idle_thread_scaling"]  = idle_thread_scaling_;
     j["idle_threads_active"] = idle_threads_active_;
     j["idle_threads_idle"]   = idle_threads_idle_;
+    j["idle_gpu_aware"]      = idle_gpu_aware_;
     j["saved_pool_urls"] = json::array();
     for (const auto& u : saved_pool_urls_)
         j["saved_pool_urls"].push_back(u);