refactor: app startup improvements

lib/core/utils/system_ui_style.dart

@@ -0,0 +1,17 @@
+import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
+
+/// Applies a single System UI overlay style after first frame to avoid flicker
+/// at startup and to align with the active theme brightness.
+void applySystemUiOverlayStyleOnce({required Brightness brightness}) {
+  // On Android 15+, avoid setting bar colors; only control icon brightness.
+  final isDark = brightness == Brightness.dark;
+  SystemChrome.setSystemUIOverlayStyle(
+    SystemUiOverlayStyle(
+      statusBarIconBrightness: isDark ? Brightness.light : Brightness.dark,
+      systemNavigationBarIconBrightness: isDark
+          ? Brightness.light
+          : Brightness.dark,
+    ),
+  );
+}