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>

lib/view/login/widgets/login_card.dart

@@ -111,6 +111,7 @@ class _LoginCardState extends State<LoginCard> {
               ),
               const SizedBox(height: 20),
               TextFormField(
+                key: const Key('login-username-field'),
                 controller: _usernameController,
                 enabled: !loading,
                 validator: _required,
@@ -125,6 +126,7 @@ class _LoginCardState extends State<LoginCard> {
               ),
               const SizedBox(height: 12),
               TextFormField(
+                key: const Key('login-password-field'),
                 controller: _passwordController,
                 focusNode: _passwordFocus,
                 enabled: !loading,
@@ -146,6 +148,7 @@ class _LoginCardState extends State<LoginCard> {
               SizedBox(
                 height: 50,
                 child: FilledButton(
+                  key: const Key('login-submit-button'),
                   onPressed: loading ? null : _submit,
                   style: FilledButton.styleFrom(
                     shape: RoundedRectangleBorder(