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/data/wallet_state.h

@@ -254,6 +254,8 @@ struct PoolMiningState {
     int64_t uptime_sec   = 0;
     double  pool_diff    = 0;
     bool    connected    = false;
+    std::time_t running_since = 0;   // wall-clock the miner was first seen running this session (0 = not running)
+    std::string last_error;          // newest xmrig log line that looks like a connection error ("" = none)
 
     // Memory/thread usage (bytes for memory)
     int64_t memory_used  = 0;