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>

.gitignore

@@ -350,3 +350,15 @@ hs_err_pid*
 
 *.idea*
 **/.DS_store
+
+# Fastlane (Play-Store-Upload)
+android/fastlane/report.xml
+android/fastlane/Preview.html
+android/fastlane/play-service-account.json
+android/fastlane/README.md.bak
+# lokale Screenshots außerhalb der Fastlane-Metadaten
+/screenshots/
+# Fastlane-Bild-PNGs sind generierte Kopien; kanonisch/versioniert liegen die
+# Screenshots unter materials/screenshots/. Nur die leeren Ordner (.gitkeep)
+# bleiben eingecheckt, damit supply die Struktur vorfindet.
+android/fastlane/metadata/android/**/images/**/*.png