added provider class for timer

lib/models/activity_timer_model.dart

@@ -0,0 +1,60 @@
+import 'dart:async';
+
+import 'package:flutter/material.dart';
+import 'package:sendtrain/models/activity_model.dart';
+
+class ActivityTimerModel with ChangeNotifier {
+  late int _activityId;
+  late int _tickCount;
+  late int _currentSet;
+  late int _totalSets;
+  late int _currentRep;
+  late int _totalReps;
+  late String _currentState;
+  late Timer? _periodicTimer;
+
+  bool _active = false;
+
+  int get tickCount => _tickCount;
+  int get currentSet => _currentSet;
+  int get currentRep => _currentRep;
+  int get totalSets => _totalSets;
+  int get totalReps => _totalReps;
+  bool get active => _active;
+
+  void setupTimer(ActivityModel activity) {
+    _activityId = activity.id;
+    _currentSet = 0;
+    _currentRep = 0;
+    _totalSets = activity.actions[0].activityActionSet.total;
+    _totalReps = activity.actions[0].activityActionSet.reps.amounts[0];
+    _tickCount = activity.actions[0].activityActionSet.rest ~/ 10000;
+  }
+
+  void startTimer(ActivityModel activity) {
+    _active = true;
+    _periodicTimer = Timer.periodic(const Duration(seconds: 1), (Timer timer) {
+      if (_tickCount == 0) {
+        if (_currentRep + 1 ==
+            activity.actions[0].activityActionSet.reps.amounts[_currentSet]) {
+          _currentSet++;
+          if (_currentSet == activity.actions[0].activityActionSet.total) {
+            timer.cancel();
+            _active = false;
+            setupTimer(activity);
+          }
+          _currentRep = 0;
+          _totalReps =
+              activity.actions[0].activityActionSet.reps.amounts[_currentSet];
+          _tickCount = activity.actions[0].activityActionSet.rest ~/ 10000;
+        } else {
+          _currentRep++;
+        }
+      } else {
+        _tickCount--;
+      }
+
+      notifyListeners();
+    });
+  }
+}