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_mode_toggle.cpp

@@ -117,7 +117,7 @@ void RenderMiningModeToggle(App* app, const WalletState& state, const MiningInfo
                 s_pool_mode = false;
                 app->settings()->setPoolMode(false);
                 app->settings()->save();
-                CancelMiningBenchmark(app);   // don't leave a pool benchmark running after switching to solo (L-11)
+                CancelMiningBenchmark(app, false);   // don't leave a pool benchmark running after switching to solo (L-11)
                 app->stopPoolMining();
             }
             if (soloHov) ImGui::SetMouseCursor(ImGuiMouseCursor_Hand);