fix(mining): restore the miner after a benchmark only if it was running

Cancelling a thread benchmark always stopped and restarted the pool
miner at its pre-benchmark thread count, even when the user had not been
mining before the benchmark began. That silently started the miner on a
plain cancel or a switch to solo mode.

- A cancel now always drops the candidate-thread miner, and restarts the
  pool miner only when the user was mining before the benchmark, read
  from a snapshot taken at the start rather than the live running state
  (the benchmark sweep transiently stops the miner between candidates,
  so a live check would drop the restore).
- Callers that must not restore the miner (leaving solo mode) pass the
  new restore flag as false.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ui/windows/mining_tab.h

@@ -22,11 +22,11 @@ void RenderMiningTab(App* app);
 bool IsMiningBenchmarkActive();
 
 /**
- * @brief Cancel a running thread benchmark and restore the miner to its pre-benchmark thread count.
+ * @brief Cancel a running thread benchmark and restore the miner only if it was running before the benchmark.
  * Safe to call when no benchmark is active (no-op). Used when leaving the Mining tab or switching to
  * solo mode so the miner isn't left stuck at a benchmark step. (L-04, L-11)
  */
-void CancelMiningBenchmark(App* app);
+void CancelMiningBenchmark(App* app, bool restoreMiner = true);
 
 } // namespace ui
 } // namespace dragonx