feat(onboarding): add theme-aware 5.0 upgrade introduction

lib/services/upgrade_intro_service.dart

@@ -0,0 +1,20 @@
+import 'package:shared_preferences/shared_preferences.dart';
+
+/// Separate from settings migrations: migrating preferences must not consume
+/// the upgrade introduction before the user has actually seen it.
+class UpgradeIntroService {
+  static const _seenKey = 'upgrade_intro_5_0_seen';
+
+  static bool shouldShow(
+    SharedPreferences preferences, {
+    required String version,
+    required bool existingInstallation,
+  }) =>
+      version.split('.').first == '5' &&
+      existingInstallation &&
+      !(preferences.getBool(_seenKey) ?? false);
+
+  static Future<void> markSeen(SharedPreferences preferences) async {
+    await preferences.setBool(_seenKey, true);
+  }
+}