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>

android/fastlane/Pluginfile

@@ -0,0 +1,2 @@
+# fastlane-Plugins (aktuell keine). Datei vorhanden, damit `fastlane` das
+# Plugin-Handling initialisiert.