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.cpp

@@ -966,7 +966,7 @@ void App::update()
         checkAutoLock();
     }
 
-    // Mine-when-idle check (runs every frame, internally rate-limited by idle detection)
+    // Mine-when-idle check (cheap on most frames: it gates itself to 1 Hz)
     checkIdleMining();
 
     // P8: Dedup rebuildAddressList — only rebuild once per frame