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/Fastfile

@@ -0,0 +1,29 @@
+default_platform(:android)
+
+# Play-Store-Auslieferung über fastlane supply. Die Screenshots erzeugt
+# ../tool/screenshots.sh (Demo-Login + integration_test) direkt in
+# metadata/android/<locale>/images/. Texte/Changelogs liegen daneben in
+# metadata/android/<locale>/.
+platform :android do
+  desc "Nur Screenshots hochladen (verändert keine Texte/Binaries)"
+  lane :upload_screenshots do
+    upload_to_play_store(
+      skip_upload_apk: true,
+      skip_upload_aab: true,
+      skip_upload_metadata: true,
+      skip_upload_changelogs: true,
+      skip_upload_images: false,
+      skip_upload_screenshots: false,
+      metadata_path: "./fastlane/metadata/android",
+    )
+  end
+
+  desc "Store-Texte, Changelogs und Screenshots hochladen (keine Binaries)"
+  lane :upload_metadata do
+    upload_to_play_store(
+      skip_upload_apk: true,
+      skip_upload_aab: true,
+      metadata_path: "./fastlane/metadata/android",
+    )
+  end
+end