feat(settings): hub bearer-token panel + auto-attach on every RPC

Studio gains a new "HUB AUTHENTICATION" panel in Settings
mirroring the existing registry-credentials panel:

- `lib/data/hub_auth_token.dart` — `~/.fai/hub-auth-token`
  helper (read / write / clear, mode 0600 on Unix). Sister
  of `RegistryToken` with the same on-disk hygiene.
- `HubService.loadPersistedEndpoint` now reads the token at
  startup and reconnects with it. `reconnect()` grew an
  `authToken:` parameter with a sentinel that distinguishes
  "keep current" from "drop". `reloadAuthToken()` is the
  one-liner Settings calls after save / clear.
- `_HubAuthTokenPanel` in `fai_settings_dialog.dart` — paste
  with show/hide toggle, save button, clear button, status
  pill ("Configured (40 chars)" / "Not set (anonymous)"),
  storage-location hint. Trimmed token length only — the
  secret never round-trips back into the UI after save.
- EN + DE ARB entries (`hubAuthToken*`) + regenerated
  `app_localizations.dart` keep the bilingual surface
  consistent.

The hub side (auth.tokens config + tower middleware) shipped
on the platform side 2026-05-28 (43a54a2). Until now an
operator had no GUI path to consume it: they had to find the
file in their home dir, paste the token by hand, and bounce
Studio. This panel closes that loop.

Bumped pubspec to 0.47.0. dart analyze clean (No issues
found!); flutter test green (11 tests pass).

Block E item 1 of 4 done. Capability-picker badges,
default_scope editor, multi-version uninstall picker follow.

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

lib/l10n/app_localizations_de.dart

@@ -1241,6 +1241,49 @@ class AppLocalizationsDe extends AppLocalizations {
     return 'Speichern fehlgeschlagen: $error';
   }
 
+  @override
+  String get hubAuthTokenHeader => 'HUB-AUTHENTIFIZIERUNG';
+
+  @override
+  String get hubAuthTokenBlurb =>
+      'Bearer-Token für die Studio-Anbindung an einen Hub mit aktivierter auth.tokens-Konfiguration (RBAC Level 2). Gespeichert in ~/.fai/hub-auth-token, Modus 0600. Studio sendet ihn als Authorization: Bearer bei jedem gRPC-Aufruf.';
+
+  @override
+  String hubAuthTokenStatusConfigured(int chars) {
+    return 'Eingerichtet ($chars Zeichen)';
+  }
+
+  @override
+  String get hubAuthTokenStatusNotSet => 'Nicht gesetzt (anonym)';
+
+  @override
+  String get hubAuthTokenFieldLabel => 'Token';
+
+  @override
+  String get hubAuthTokenFieldHint =>
+      'Token aus dem auth.tokens-Eintrag des Hub-Operators einfügen';
+
+  @override
+  String get hubAuthTokenSaveButton => 'Speichern';
+
+  @override
+  String get hubAuthTokenClearButton => 'Entfernen';
+
+  @override
+  String get hubAuthTokenStorageHint =>
+      'Lokal gespeichert in ~/.fai/hub-auth-token. Studio verbindet nach dem Speichern automatisch neu, der Token wirkt sofort.';
+
+  @override
+  String get hubAuthTokenSavedToast => 'Hub-Token gespeichert.';
+
+  @override
+  String get hubAuthTokenClearedToast => 'Hub-Token entfernt.';
+
+  @override
+  String hubAuthTokenSaveFailedToast(String error) {
+    return 'Speichern fehlgeschlagen: $error';
+  }
+
   @override
   String get maintenanceHeader => 'HUB-WARTUNG';