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>

l10n.yaml

@@ -0,0 +1,13 @@
+# `flutter gen-l10n` config. Generates a typed
+# `AppLocalizations` accessor from the ARB files in
+# `lib/l10n/`. Re-run on every ARB edit:
+#
+#   flutter gen-l10n
+#
+# (or just `flutter pub get` / `flutter run` — gen-l10n is
+# wired into the build pipeline because pubspec.yaml has
+# `flutter.generate: true`.)
+arb-dir: lib/l10n
+template-arb-file: app_en.arb
+output-localization-file: app_localizations.dart
+output-class: AppLocalizations