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.h

@@ -601,6 +601,10 @@ private:
     // Daemon restart (e.g. after changing debug log categories)
     std::atomic<bool> daemon_restarting_{false};
 
+    // Set by the deleteBlockchainData worker (item count); the main loop surfaces a completion toast
+    // and resets it to -1. Atomic because the worker thread writes it and the UI thread reads/clears it.
+    std::atomic<int> pending_delete_result_{-1};
+
     // Encryption state check timeout
     float encryption_check_timer_ = 0.0f;
 
@@ -702,6 +706,10 @@ private:
     // Mining toggle guard (prevents concurrent setgenerate calls)
     std::atomic<bool> mining_toggle_in_progress_{false};
 
+    // True from a successful startPoolMining() until the miner is confirmed connected/hashing in the
+    // poll — drives the "connecting…" → "connected" feedback for pool mining (which has a connect delay).
+    std::atomic<bool> pool_starting_{false};
+
     // Auto-shield guard (prevents concurrent auto-shield operations)
     std::atomic<bool> auto_shield_pending_{false};