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.cpp

@@ -55,17 +55,16 @@ bool IsMiningBenchmarkActive() {
     return s_benchmark.active();
 }
 
-void CancelMiningBenchmark(App* app) {
+void CancelMiningBenchmark(App* app, bool restoreMiner) {
     if (!s_benchmark.active()) return;
-    const int restoreThreads = s_benchmark.prev_threads;
+    const ThreadBenchmarkCancel plan = PlanThreadBenchmarkCancel(s_benchmark);   // read BEFORE reset() zeroes both fields
     s_benchmark.reset();
-    // Restore the miner to its pre-benchmark thread count. A benchmark runs in pool mode, so restore
-    // regardless of the instantaneous running state — the sweep may be mid inter-candidate stop, where an
-    // isPoolMinerRunning() check would be transiently false and silently drop the restart. (L-04, L-11)
-    if (app && restoreThreads > 0) {
-        app->stopPoolMining();
-        app->startPoolMining(restoreThreads);
-    }
+    if (!app) return;
+    // Always drop the candidate-thread miner. Restore only if the user was mining before the benchmark, using the
+    // was_pool_running SNAPSHOT (not the live state: the sweep may be mid inter-candidate stop). (L-04, L-11)
+    app->stopPoolMining();
+    if (restoreMiner && plan.restartMiner)
+        app->startPoolMining(plan.restartThreads);
 }
 
 // Miner-update version check (one shot per session): fetches the latest DRG-XMRig release tag in