feat(mining): persistent start/stop error caption + timeout-aware pool status

- A mining start/stop/pool failure now sets a persistent App::mining_last_error_
  (cleared on the next attempt), rendered as a red caption with "Press Mine to
  try again" below the Mine button (plus a Dismiss, and a Windows-Defender
  "How to fix" on an AV block), so a crashed/blocked/failed miner is no longer
  just a button that "did nothing" once its toast faded. Two more hardcoded
  English strings (pool-payout-invalid, AV-blocked) moved to i18n.
- Pool connection status is now a persistent "Status" row in the pool stats
  card (PoolMiningState.running_since / last_error): it flips from "Connecting…"
  to "Can't reach pool — check the URL" after a 30s timeout instead of sitting
  on red forever, and no longer vanishes once logs arrive. The old transient
  banner in mining_tab.cpp was removed.

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

src/app.h

@@ -389,6 +389,11 @@ public:
     void startMining(int threads);
     void stopMining();
     bool isMiningToggleInProgress() const { return mining_toggle_in_progress_.load(std::memory_order_relaxed); }
+    // Persistent mining error (last start/stop/pool failure) so a failed miner isn't just a fading toast.
+    const std::string& miningLastError() const { return mining_last_error_; }
+    bool miningErrorIsAntivirus() const { return mining_error_av_; }
+    void clearMiningError() { mining_last_error_.clear(); mining_error_av_ = false; }
+    void showAntivirusHelp() { pending_antivirus_dialog_ = true; }   // Windows-only dialog; inert elsewhere
 
     // Pool mining (xmrig)
     void startPoolMining(int threads);
@@ -1745,6 +1750,10 @@ private:
 
     // Mining toggle guard (prevents concurrent setgenerate calls)
     std::atomic<bool> mining_toggle_in_progress_{false};
+    // Mining error state (main-thread only — every writer is in a MainCb or render).
+    std::string mining_last_error_;
+    bool mining_error_av_ = false;
+    void setMiningError(std::string msg, bool av = false) { mining_last_error_ = std::move(msg); mining_error_av_ = av; }
 
     // 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).