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_en.dart

@@ -1256,6 +1256,49 @@ class AppLocalizationsEn extends AppLocalizations {
     return 'Could not save: $error';
   }
 
+  @override
+  String get hubAuthTokenHeader => 'HUB AUTHENTICATION';
+
+  @override
+  String get hubAuthTokenBlurb =>
+      'Bearer token used to authenticate Studio against a hub that has auth.tokens: configured (RBAC Level 2). Stored at ~/.fai/hub-auth-token, mode 0600. Studio sends it as Authorization: Bearer on every gRPC call.';
+
+  @override
+  String hubAuthTokenStatusConfigured(int chars) {
+    return 'Configured ($chars chars)';
+  }
+
+  @override
+  String get hubAuthTokenStatusNotSet => 'Not set (anonymous)';
+
+  @override
+  String get hubAuthTokenFieldLabel => 'Token';
+
+  @override
+  String get hubAuthTokenFieldHint =>
+      'Paste a token from the hub operator\'s auth.tokens entry';
+
+  @override
+  String get hubAuthTokenSaveButton => 'Save';
+
+  @override
+  String get hubAuthTokenClearButton => 'Clear';
+
+  @override
+  String get hubAuthTokenStorageHint =>
+      'Saved locally to ~/.fai/hub-auth-token. Studio reconnects after save so the new token takes effect immediately.';
+
+  @override
+  String get hubAuthTokenSavedToast => 'Hub auth token saved.';
+
+  @override
+  String get hubAuthTokenClearedToast => 'Hub auth token cleared.';
+
+  @override
+  String hubAuthTokenSaveFailedToast(String error) {
+    return 'Could not save: $error';
+  }
+
   @override
   String get maintenanceHeader => 'HUB MAINTENANCE';