feat: add intelligent HAProxy validation error display in UI

- Add haproxy_error_parser.py: Parses HAProxy validation errors with
  confidence scoring, extracts entity type/name, line number, error type
- Add ValidationErrorModal.js: Rich modal with parsed error summary,
  quick fix suggestions, and manual troubleshooting guide
- Update cluster.py: Integrate error parser into agent-sync and
  config-versions endpoints with graceful fallback
- Update ApplyManagement.js: Add validation error banner with quick
  navigation buttons and error detail modal
- Update FrontendManagement.js & BackendServers.js: Handle URL params
  for deep-linking to entity edit forms with field highlighting

Enables users to see actionable validation failure details directly
in the UI without needing server access for debugging.

frontend/src/components/FrontendManagement.js

@@ -12,7 +12,7 @@ import {
   WarningOutlined, SearchOutlined, HistoryOutlined, PlayCircleOutlined, LoadingOutlined
 } from '@ant-design/icons';
 import axios from 'axios';
-import { useNavigate } from 'react-router-dom';
+import { useNavigate, useLocation } from 'react-router-dom';
 import { useCluster } from '../contexts/ClusterContext';
 import { VersionHistory } from './VersionHistory';
 
@@ -60,6 +60,7 @@ const { TextArea } = Input;
 const FrontendManagement = () => {
   const { selectedCluster } = useCluster();
   const navigate = useNavigate();
+  const location = useLocation();
   const [frontends, setFrontends] = useState([]);
   const [backends, setBackends] = useState([]);
   const [sslCertificates, setSslCertificates] = useState([]);
@@ -162,6 +163,37 @@ const FrontendManagement = () => {
     checkPendingChanges();
   }, [selectedCluster]);
 
+  // Handle URL parameters for quick edit navigation (from validation error modal)
+  useEffect(() => {
+    const params = new URLSearchParams(location.search);
+    const editId = params.get('edit');
+    const highlightField = params.get('highlight');
+    
+    if (editId && frontends.length > 0) {
+      const frontendToEdit = frontends.find(f => f.id === parseInt(editId));
+      if (frontendToEdit) {
+        // Open edit modal for the frontend
+        setEditingFrontend(frontendToEdit);
+        form.setFieldsValue({
+          ...frontendToEdit,
+          ssl_enabled: frontendToEdit.ssl_enabled || false,
+          ssl_certificate_ids: frontendToEdit.ssl_certificate_ids || [],
+          acl_rules: frontendToEdit.acl_rules || [],
+          use_backend_rules: frontendToEdit.use_backend_rules || [],
+        });
+        setModalVisible(true);
+        
+        // Clear URL params after opening modal
+        navigate(location.pathname, { replace: true });
+        
+        // Show notification about which field to check
+        if (highlightField) {
+          message.info(`Please check the "${highlightField}" field - it may have caused a validation error.`, 5);
+        }
+      }
+    }
+  }, [location.search, frontends]);
+
   const fetchFrontends = async () => {
     // CRITICAL FIX: Don't fetch if no cluster selected to prevent race condition
     // Same race condition as BackendServers - prevents all frontends from appearing