fix: Tier-2 remaining mediums — maintenance/mining feedback + force-quit hang detection

The last four (more involved) robustness items from the audit:

- deleteBlockchainData: post the deleted-item count to the main thread (via an
  atomic, since Notifications isn't thread-safe) and show a completion toast
  ("Blockchain data deleted (N items). Daemon restarting…") — previously the
  result was only logged.
- Pool mining: pool start has a connect delay with no feedback; announce
  "Starting pool miner — connecting…" on a successful start and "Pool miner
  connected and hashing." once the poll confirms it (contained pool_starting_
  flag; the shared mining-toggle state machine is untouched).
- Force Quit (shutdown screen): gate it on the status text having STALLED (a
  genuine hang) rather than a bare 10s timer — with a 20s hard-ceiling backstop —
  and show a state-aware caution naming the stuck step (force-quitting mid daemon
  flush risks the chainstate).
- Benchmark: require a confirming second click that first builds candidates to
  estimate the duration ("Benchmark takes ~Ns and interrupts mining"), instead
  of interrupting mining immediately.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/app_network.cpp

@@ -1949,6 +1949,10 @@ void App::startPoolMining(int threads)
         } else {
             ui::Notifications::instance().error("Failed to start pool miner: " + err);
         }
+    } else {
+        // Miner spawned — it still needs a few seconds to connect to the pool and start hashing.
+        pool_starting_.store(true, std::memory_order_relaxed);
+        ui::Notifications::instance().info("Starting pool miner — connecting to the pool…");
     }
 }