Update strings and refactor weight management

- Added new string constants for BMI and target weight in `StringsManager`.
- Refactored `CustomAppBar` to conditionally display the back button based on navigation state.
- Updated `WeightCubit` to simplify weight update logic.
- Modified `WeightRulerSection` to use the new string constant for BMI label.

Author: AbdoWa7eed

lib/core/res/strings_manager.dart

@@ -74,4 +74,7 @@ abstract class AppStrings {
   static const String gallery = 'Gallery';
   static const String camera = 'Camera';
   static const String cancel = 'Cancel';
+  static const String yourBmi = 'Your BMI';
+  static const String yourTargetWeight = "What's your target weight?";
+  
 }

lib/core/widgets/custom_app_bar.dart

@@ -6,7 +6,7 @@ class CustomAppBar extends StatelessWidget implements PreferredSizeWidget {
   final String? title;
   final VoidCallback? onBack;
   final List<Widget>? actions;
-  final TextStyle?titleStyle;
+  final TextStyle? titleStyle;
 
   const CustomAppBar({
     super.key,
@@ -21,10 +21,13 @@ class CustomAppBar extends StatelessWidget implements PreferredSizeWidget {
     return AppBar(
       title: Text(title ?? '', style: titleStyle ?? StylesManager.regular18),
       centerTitle: true,
-      leading: IconButton(
-        icon: const Icon(Icons.arrow_back_ios_new_rounded),
-        onPressed: onBack ?? () => context.pop(),
-      ),
+      leading:
+          context.canPop()
+              ? IconButton(
+                icon: const Icon(Icons.arrow_back_ios_new_rounded),
+                onPressed: onBack ?? () => context.pop(),
+              )
+              : null,
       elevation: 0,
       actions: actions,
       backgroundColor: Colors.transparent,

lib/features/trainee_measurements/presentation/cubits/weight/weight_cubit.dart

@@ -4,7 +4,7 @@ import 'package:graduation_project/features/trainee_measurements/domain/entities
 import 'package:graduation_project/features/trainee_measurements/presentation/cubits/weight/weight_state.dart';
 import 'package:injectable/injectable.dart';
 
-@injectable
+@lazySingleton
 class WeightCubit extends Cubit<WeightState> {
   final double _heightInCm;
   final Gender _gender;
@@ -13,9 +13,8 @@ class WeightCubit extends Cubit<WeightState> {
     : super(const WeightState());
 
   void updateWeight(int value) {
-    final weightInKg = state.isKg ? value.toDouble() : value / 2.20462;
-    final bmi = _calculateBmi(weightInKg, _heightInCm);
-    emit(state.copyWith(weight: weightInKg, bmi: bmi));
+    final bmi = _calculateBmi(value.toDouble(), _heightInCm);
+    emit(state.copyWith(weight: value.toDouble(), bmi: bmi));
   }
 
   void toggleUnit(bool isKg) {

lib/features/trainee_measurements/presentation/widgets/weight/weight_picker_widget.dart

@@ -38,7 +38,7 @@ class WeightRulerSection extends StatelessWidget {
             const SizedBox(height: AppSize.s40),
             MeasurementFeedbackWidget(
               value: state.bmi.value.toStringAsFixed(1),
-              label: 'Your BMI',
+              label: AppStrings.yourBmi,
               status: state.bmi.category.displayName,
               message: cubit.bmiMessage,
               statusColor: state.bmi.category.color,