refactor(ui): restructure widget hierarchy to use Stack for improved layout

lib/main.dart

@@ -26,7 +26,9 @@ class MyApp extends ConsumerWidget {
       themeMode: themeState.themeMode,
       routerConfig: router,
       builder: (context, child) {
-        return Container(
+        return Stack(
+          children: [
+            Container(
               decoration: BoxDecoration(
                 gradient: AppColors.grayGlossyGradient,
                 boxShadow: [
@@ -37,7 +39,9 @@ class MyApp extends ConsumerWidget {
                   ),
                 ],
               ),
-          child: child,
+            ),
+            child!,
+          ],
         );
       },
     );