perf(mining): run the mine-when-idle check at 1 Hz, probe the GPU sparingly

checkIdleMining ran every frame: an X11 idle query and, with GPU-aware
idle detection, an nvidia-smi launch on the UI thread per frame.

- The check runs once a second, since its decisions work on a seconds
  scale (the idle delay is tens of seconds or more). A miner it started
  or scaled up still yields on the first frame that sees the user back:
  that path skips the gate, because frames can be seconds apart while
  the app idles.
- The GPU is probed only when input is idle and the result can change
  a decision, at most every 5 s, with hysteresis (busy at 10%+, clear
  below 5%) so a GPU hovering near 10% can't keep restarting the miner.
- Thread scaling returns early when no miner is running or a toggle is
  in flight.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app.h

@@ -1604,6 +1604,9 @@ private:
 
     // Mine-when-idle: auto-start/stop mining based on system idle state
     bool idle_mining_active_ = false;  // true when mining was auto-started by idle detection
+    std::chrono::steady_clock::time_point idle_mining_next_check_{};  // checkIdleMining runs at 1 Hz
+    std::chrono::steady_clock::time_point idle_gpu_next_sample_{};    // GPU probe at most every 5 s
+    bool idle_gpu_busy_ = false;       // last GPU-aware verdict (with hysteresis)
     bool idle_scaled_to_idle_ = false; // true when threads have been scaled up for idle
 
     // Private methods - rendering