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>

test_driver/integration_test.dart

@@ -0,0 +1,21 @@
+import 'dart:io';
+
+import 'package:integration_test/integration_test_driver_extended.dart';
+
+/// Host-seitiger Treiber für den Screenshot-Lauf. Schreibt jeden von
+/// `binding.takeScreenshot(name)` gelieferten PNG-Frame nach
+/// `$SCREENSHOT_OUT_DIR/<name>.png` (Default: ./screenshots). Das Zielverzeichnis
+/// setzt tool/screenshots.sh pro Gerätegröße auf das passende Fastlane-Bildverzeichnis.
+Future<void> main() async {
+  final outDir = Platform.environment['SCREENSHOT_OUT_DIR'] ?? 'screenshots';
+
+  await integrationDriver(
+    onScreenshot: (name, bytes, [args]) async {
+      final file = File('$outDir/$name.png');
+      await file.create(recursive: true);
+      await file.writeAsBytes(bytes);
+      stdout.writeln('Screenshot gespeichert: ${file.path}');
+      return true;
+    },
+  );
+}