add build step for flutter web, add persistent pagination in the traction list

lib/components/pages/traction/traction_page.dart

@@ -37,6 +37,9 @@ class _TractionPageState extends State<TractionPage> {
   final Map<String, TextEditingController> _dynamicControllers = {};
   final Map<String, String?> _enumSelections = {};
   bool _restoredFromPrefs = false;
+  static const int _pageSize = 100;
+  int _lastTractionOffset = 0;
+  String? _lastQuerySignature;
 
   @override
   void initState() {
@@ -59,6 +62,9 @@ class _TractionPageState extends State<TractionPage> {
   Future<void> _initialLoad() async {
     final data = context.read<DataService>();
     await _restoreSearchState();
+    if (_lastTractionOffset == 0 && data.traction.length > _pageSize) {
+      _lastTractionOffset = data.traction.length - _pageSize;
+    }
     data.fetchClassList();
     data.fetchEventFields();
     await _refreshTraction();
@@ -103,7 +109,29 @@ class _TractionPageState extends State<TractionPage> {
         dynamicFieldsUsed;
   }
 
-  Future<void> _refreshTraction({bool append = false}) async {
+  String _tractionQuerySignature(
+    Map<String, dynamic> filters,
+    bool hadOnly,
+  ) {
+    final sortedKeys = filters.keys.toList()..sort();
+    final filterSignature = sortedKeys
+        .map((key) => '$key=${filters[key]}')
+        .join('|');
+    final classQuery = (_selectedClass ?? _classController.text).trim();
+    return [
+      'class=$classQuery',
+      'number=${_numberController.text.trim()}',
+      'name=${_nameController.text.trim()}',
+      'mileageFirst=$_mileageFirst',
+      'hadOnly=$hadOnly',
+      'filters=$filterSignature',
+    ].join(';');
+  }
+
+  Future<void> _refreshTraction({
+    bool append = false,
+    bool preservePosition = true,
+  }) async {
     final data = context.read<DataService>();
     final filters = <String, dynamic>{};
     final name = _nameController.text.trim();
@@ -118,15 +146,49 @@ class _TractionPageState extends State<TractionPage> {
       }
     });
     final hadOnly = !_hasFilters;
+    final signature = _tractionQuerySignature(filters, hadOnly);
+    final queryChanged =
+        _lastQuerySignature != null && signature != _lastQuerySignature;
+    _lastQuerySignature = signature;
+
+    if (queryChanged && !append) {
+      _lastTractionOffset = 0;
+    }
+
+    final shouldPreservePosition = preservePosition &&
+        !append &&
+        !queryChanged &&
+        _lastTractionOffset > 0;
+
+    int limit;
+    int offset;
+    if (append) {
+      offset = data.traction.length;
+      limit = _pageSize;
+      _lastTractionOffset = offset;
+    } else if (shouldPreservePosition) {
+      offset = 0;
+      limit = _pageSize + _lastTractionOffset;
+    } else {
+      offset = 0;
+      limit = _pageSize;
+    }
+
     await data.fetchTraction(
       hadOnly: hadOnly,
       locoClass: _selectedClass ?? _classController.text.trim(),
       locoNumber: _numberController.text.trim(),
-      offset: append ? data.traction.length : 0,
+      offset: offset,
+      limit: limit,
       append: append,
       filters: filters,
       mileageFirst: _mileageFirst,
     );
+
+    if (!append && !shouldPreservePosition) {
+      _lastTractionOffset = 0;
+    }
+
     await _persistSearchState();
   }
 

lib/components/pages/traction/traction_persistence.dart

@@ -39,6 +39,16 @@ extension _TractionPersistence on _TractionPageState {
           enumValues[entry.key.toString()] = entry.value?.toString();
         }
       }
+      final lastOffsetRaw = decoded['lastOffset'];
+      if (lastOffsetRaw is int) {
+        _lastTractionOffset = lastOffsetRaw;
+      } else if (lastOffsetRaw is num) {
+        _lastTractionOffset = lastOffsetRaw.toInt();
+      }
+      final lastSig = decoded['querySignature']?.toString();
+      if (lastSig != null && lastSig.isNotEmpty) {
+        _lastQuerySignature = lastSig;
+      }
 
       for (final entry in dynamicValues.entries) {
         _dynamicControllers.putIfAbsent(
@@ -76,6 +86,8 @@ extension _TractionPersistence on _TractionPageState {
       'showAdvancedFilters': _showAdvancedFilters,
       'dynamic': _dynamicControllers.map((k, v) => MapEntry(k, v.text)),
       'enum': _enumSelections,
+      'lastOffset': _lastTractionOffset,
+      'querySignature': _lastQuerySignature,
     };
 
     try {