feat(studio): app-wide i18n via flutter_localizations (v0.24.0)

Replaces the Store-only DE/EN toggle with an app-wide one
parked in the sidebar footer next to the theme button.
Pressing it flips every translated string at once: nav
labels, page titles, common buttons, the bilingual
store-index content.

Implementation:
- Adds `flutter_localizations` + `intl` to pubspec, plus
  `flutter.generate: true` so `flutter gen-l10n` runs in the
  build pipeline.
- ARB sources at `lib/l10n/app_en.arb` and `app_de.arb`. The
  EN file is the template; DE carries the German strings.
  Initial coverage: navigation, common buttons, page titles,
  channels / store / audit / modules / approvals headers,
  hub-unreachable copy, MCP + n8n panel headers + hints.
  Rest of the UI strings are still English-literal — those
  fall in incrementally as we touch each surface.
- Generated `AppLocalizations` lives at
  `lib/l10n/app_localizations*.dart` (regenerated via
  `flutter gen-l10n` on every ARB edit).
- `StudioAppState` gains `localeNotifier` alongside
  `modeNotifier`; persisted via SharedPreferences key
  `locale.code`.
- Sidebar `_LanguageToggle` reads/writes through the
  notifier. The Store's per-page locale state is gone:
  `_locale` now reads `Localizations.localeOf(context)
  .languageCode`, so the bilingual store-index content
  follows the global setting without a second toggle.
- `_NavPage.label` becomes `_NavPage.id` + `labelOf(context)`;
  Cmd+K palette and Sidebar both read the localized label.

Out of scope this iteration: localizing the remaining
~80% of UI strings (Settings dialog labels, Store search
hint, error messages). Those land incrementally — the i18n
infrastructure now means each is a one-line ARB edit + one
call-site swap.

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

test/widget_test.dart

@@ -1,5 +1,6 @@
 // Smoke test: app boots and shows every navigation destination.
 
+import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:fai_studio/data/hub.dart';
 import 'package:fai_studio/main.dart';
@@ -8,7 +9,10 @@ void main() {
   testWidgets('Studio shell shows every navigation destination',
       (tester) async {
     await tester.pumpWidget(
-      const StudioApp(initialThemeMode: ThemeModeValue.system),
+      const StudioApp(
+        initialThemeMode: ThemeModeValue.system,
+        initialLocale: Locale('en'),
+      ),
     );
 
     expect(find.text('F∆I Studio'), findsOneWidget);