refactor(skeletons): update children type annotations and improve consistency in skeleton widgets

apps/mobile/packages/features/client/billing/lib/src/presentation/widgets/billing_page_skeleton/invoice_card_skeleton.dart

@@ -16,10 +16,10 @@ class InvoiceCardSkeleton extends StatelessWidget {
       ),
       child: Column(
         crossAxisAlignment: CrossAxisAlignment.start,
-        children: [
+        children: <Widget>[
           Row(
             mainAxisAlignment: MainAxisAlignment.spaceBetween,
-            children: [
+            children: <Widget>[
               UiShimmerBox(
                 width: 72,
                 height: 24,
@@ -35,10 +35,10 @@ class InvoiceCardSkeleton extends StatelessWidget {
           const SizedBox(height: UiConstants.space4),
           Row(
             mainAxisAlignment: MainAxisAlignment.spaceBetween,
-            children: [
+            children: <Widget>[
               const Column(
                 crossAxisAlignment: CrossAxisAlignment.start,
-                children: [
+                children: <Widget>[
                   UiShimmerLine(width: 80, height: 10),
                   SizedBox(height: UiConstants.space1),
                   UiShimmerLine(width: 100, height: 18),

apps/mobile/packages/features/client/billing/lib/src/presentation/widgets/invoices_list_skeleton.dart

@@ -15,7 +15,7 @@ class InvoicesListSkeleton extends StatelessWidget {
       child: Padding(
         padding: const EdgeInsets.all(UiConstants.space5),
         child: Column(
-          children: List.generate(4, (int index) {
+          children: List<Widget>.generate(4, (int index) {
             return Padding(
               padding: const EdgeInsets.only(bottom: UiConstants.space4),
               child: Container(