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>

lib/data/hub.dart

@@ -6,6 +6,7 @@
 // protobuf imports.
 
 import 'package:fai_dart_sdk/fai_dart_sdk.dart';
+import 'package:flutter/widgets.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 
 class HubService {
@@ -53,6 +54,7 @@ class HubService {
   }
 
   static const _kThemeKey = 'theme.mode';
+  static const _kLocaleKey = 'locale.code';
 
   /// Read the persisted theme mode (system / light / dark) for
   /// initial app startup. Defaults to system.
@@ -68,6 +70,22 @@ class HubService {
     await prefs.setString(_kThemeKey, mode.wire);
   }
 
+  /// Read the persisted UI locale. Defaults to English when
+  /// nothing is stored — operators on a fresh install land in
+  /// English; the sidebar toggle flips to German on demand.
+  Future<Locale> loadLocale() async {
+    final prefs = await SharedPreferences.getInstance();
+    final raw = prefs.getString(_kLocaleKey);
+    if (raw == 'de') return const Locale('de');
+    return const Locale('en');
+  }
+
+  /// Persist the locale code (`en` / `de`).
+  Future<void> saveLocale(Locale locale) async {
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setString(_kLocaleKey, locale.languageCode);
+  }
+
   Future<bool> healthy() => _client.healthy();
 
   Future<List<ModuleSummary>> listModules() async {