test: GUIDE_SHOTS_LOCALE for the guide harness (EN-parity proof runs)

The screenshot harness already had GUIDE_SHOTS_THEME; add a matching
GUIDE_SHOTS_LOCALE=en so the English wizard/pages can be captured for
parity checks. The CTA finder accepts both locales. Verified: EN run
renders the fully-translated wizard with no leaked German and no
layout breaks.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

integration_test/guide_shots_test.dart

@@ -79,6 +79,12 @@ ThemeModeValue get _theme =>
     ? ThemeModeValue.light
     : ThemeModeValue.dark;
 
+/// Guide images ship German; GUIDE_SHOTS_LOCALE=en exists for
+/// English-parity proof runs.
+Locale get _locale => Locale(
+  Platform.environment['GUIDE_SHOTS_LOCALE'] == 'en' ? 'en' : 'de',
+);
+
 /// Rasterize the app's RepaintBoundary into `<outDir>/<name>.png`.
 Future<void> _shot(WidgetTester tester, String name) async {
   // Two extra frames so ripples/route transitions settle visually.
@@ -188,7 +194,7 @@ void main() {
         key: _shotKey,
         child: StudioApp(
           initialThemeMode: _theme,
-          initialLocale: const Locale('de'),
+          initialLocale: _locale,
         ),
       ),
     );
@@ -237,7 +243,9 @@ void main() {
     // it requires switching to a second, isolated hub instance.
     shell.navigateTo('welcome');
     await _pumpFrames(tester);
-    final cta = find.text('In 3 Fragen loslegen');
+    final cta = find.textContaining(
+      _locale.languageCode == 'en' ? '3 questions' : '3 Fragen',
+    );
     if (cta.evaluate().isNotEmpty) {
       await tester.tap(cta.first);
       await _pumpFrames(tester);