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/ui/windows/mining_tab.cpp

@@ -335,24 +335,9 @@ static void RenderMiningTabContent(App* app)
                          isMiningActive, s_pool_mode, availWidth, glassSpec, sHdr, gapOver);
 
     // ================================================================
-    // POOL CONNECTION STATUS — inline indicator (pool mode, no log)
-    // ================================================================
-    if (s_pool_mode && state.pool_mining.log_lines.empty() && state.pool_mining.xmrig_running) {
-        ImFont* iconFont = Type().iconSmall();
-        const char* dotIcon = ICON_MD_CIRCLE;
-        ImU32 dotCol = state.pool_mining.connected ? WithAlpha(Success(), 200) : WithAlpha(Error(), 200);
-        const char* statusText = state.pool_mining.connected
-            ? (state.pool_mining.pool_url.empty() ? TR("mining_connected") : state.pool_mining.pool_url.c_str())
-            : TR("mining_connecting");
-
-        ImVec2 pos = ImGui::GetCursorScreenPos();
-        ImVec2 dotSz = iconFont->CalcTextSizeA(iconFont->LegacySize, 1000.0f, 0.0f, dotIcon);
-        dl->AddText(iconFont, iconFont->LegacySize, pos, dotCol, dotIcon);
-        dl->AddText(capFont, capFont->LegacySize,
-                    ImVec2(pos.x + dotSz.x + 4 * dp, pos.y + (dotSz.y - capFont->LegacySize) * 0.5f),
-                    OnSurfaceMedium(), statusText);
-        ImGui::Dummy(ImVec2(availWidth, capFont->LegacySize + 4 * dp));
-    }
+    // (Pool connection status now lives as a persistent, timeout-aware row in RenderLeftPoolCard
+    //  (mining_stats.cpp) — the old transient "Connecting" banner that vanished once logs arrived and
+    //  sat on red forever on failure was removed.)
 }
 
 } // namespace ui