feat: add dead-man's switch overlay and console warning

- Poll /health every 5s; show full-screen overlay after 2 consecutive
  failures telling the user the console was closed
- Auto-hide overlay when backend comes back online
- Added to base.html (all main pages) and setup.html (first-run/settings)
- setup.html: suppress false-positive overlay during intentional restart
  via awaitingRestart flag
- setup.html: add amber warning banner that the console must stay open
- .gitignore: block *_import.sql and *_export.csv to prevent future
  accidental commits of real supplier/pricing data

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/templates/base.html

@@ -50,6 +50,25 @@
 
     <div id="toast" class="fixed bottom-4 right-4 z-50"></div>
 
+    <!-- Dead-man's switch overlay: shown when backend process stops responding -->
+    <div id="backend-offline-overlay" class="hidden fixed inset-0 z-[9999] bg-black/80 flex items-center justify-center p-6">
+        <div class="bg-white rounded-xl shadow-2xl max-w-md w-full p-8 text-center">
+            <div class="text-red-500 mb-4">
+                <svg class="w-16 h-16 mx-auto" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+                    <path stroke-linecap="round" stroke-linejoin="round" stroke-width="1.5" d="M9.75 17L9 20l-1 1h8l-1-1-.75-3M3 13h18M5 17h14a2 2 0 002-2V5a2 2 0 00-2-2H5a2 2 0 00-2 2v10a2 2 0 002 2z"/>
+                </svg>
+            </div>
+            <h2 class="text-xl font-bold text-gray-900 mb-3">Приложение остановлено</h2>
+            <p class="text-gray-600 mb-4">Консольное окно QuoteForge было закрыто — без него программа не работает.</p>
+            <div class="bg-amber-50 border border-amber-200 rounded-lg p-3 mb-6 text-sm text-amber-800">
+                Запустите программу заново и нажмите «Обновить страницу».
+            </div>
+            <button onclick="window.location.reload()" class="w-full px-6 py-3 bg-blue-600 text-white rounded-lg hover:bg-blue-700 font-medium">
+                Обновить страницу
+            </button>
+        </div>
+    </div>
+
     <!-- Sync Info Modal -->
     <div id="sync-modal" class="fixed inset-0 bg-black bg-opacity-50 flex items-center justify-center z-50 hidden p-4">
         <div class="bg-white rounded-lg shadow-xl max-w-lg w-full max-h-[90vh] flex flex-col">
@@ -493,6 +512,35 @@
         loadDBUser();
         checkWritePermission();
 
+        // Dead-man's switch: detect if the backend process has stopped
+        (function() {
+            const POLL_MS = 5000;
+            const FAIL_THRESHOLD = 2;
+            let failCount = 0;
+
+            async function checkBackend() {
+                try {
+                    const ctrl = new AbortController();
+                    const timer = setTimeout(() => ctrl.abort(), 3000);
+                    const resp = await fetch('/health', { signal: ctrl.signal });
+                    clearTimeout(timer);
+                    if (resp.ok) {
+                        failCount = 0;
+                        document.getElementById('backend-offline-overlay').classList.add('hidden');
+                    } else {
+                        failCount++;
+                    }
+                } catch (_) {
+                    failCount++;
+                }
+                if (failCount >= FAIL_THRESHOLD) {
+                    document.getElementById('backend-offline-overlay').classList.remove('hidden');
+                }
+            }
+
+            setInterval(checkBackend, POLL_MS);
+        })();
+
         // Load last sync time - removed since dropdown is gone
         // document.addEventListener('DOMContentLoaded', loadLastSyncTime);
     </script>