Add Fastlane Play Store integration with automated screenshots

Fastlane supply config (android/fastlane) with de-DE metadata structure,
integration_test-driven screenshot automation (screenshot_test.dart,
test_driver, tool/screenshots.sh) using stable keys on the login form,
refreshed materials/screenshots (phone + 7-inch) and app banner/symbol.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

pubspec.yaml

@@ -98,6 +98,11 @@ dependencies:
 dev_dependencies:
   flutter_test:
     sdk: flutter
+  # Screenshot-Automatisierung für den Play Store: treibt den Demo-Login über
+  # flutter drive und ruft binding.takeScreenshot pro Hauptscreen auf
+  # (integration_test/ + test_driver/, orchestriert von tool/screenshots.sh).
+  integration_test:
+    sdk: flutter
   fake_async: ^1.3.1
 
   flutter_launcher_icons: ^0.14.3