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/Appfile

@@ -0,0 +1,6 @@
+package_name("eu.mhsl.marianum.mobile.client")
+
+# Service-Account-JSON für die Google Play Developer API. Pfad über die
+# Umgebungsvariable SUPPLY_JSON_KEY setzen (die Datei selbst wird per
+# .gitignore nicht eingecheckt).
+json_key_file(ENV["SUPPLY_JSON_KEY"] || "fastlane/play-service-account.json")