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/data/hub.dart

@@ -12,6 +12,7 @@ import 'package:flutter/widgets.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 
 import 'flow_output.dart';
+import 'hub_auth_token.dart';
 export 'flow_output.dart';
 
 class HubService {
@@ -30,34 +31,54 @@ class HubService {
   static const _kPortKey = 'hub.port';
   static const _kSecureKey = 'hub.secure';
 
-  /// Read persisted endpoint and reconnect if it differs from
-  /// the default. Called once at app start.
+  /// Sentinel distinguishing "caller did not pass authToken"
+  /// from "caller passed null to drop the token".
+  static const Object _unset = Object();
+
+  /// Read persisted endpoint + auth token, then reconnect.
+  /// Called once at app start; safe to call again after the
+  /// operator updates the token in Settings.
   Future<void> loadPersistedEndpoint() async {
     final prefs = await SharedPreferences.getInstance();
     final host = prefs.getString(_kHostKey);
     final port = prefs.getInt(_kPortKey);
     final secure = prefs.getBool(_kSecureKey);
-    if (host == null) return;
+    final token = await HubAuthToken.read();
+    if (host == null && token == null) return;
     final endpoint = HubEndpoint(
-      host: host,
-      port: port ?? 50051,
-      secure: secure ?? false,
+      host: host ?? _client.endpoint.host,
+      port: port ?? _client.endpoint.port,
+      secure: secure ?? _client.endpoint.secure,
     );
-    if (endpoint.toString() != _client.endpoint.toString()) {
-      await reconnect(endpoint);
-    }
+    await reconnect(endpoint, authToken: token);
   }
 
   /// Reconnect to a new endpoint and persist for next launch.
-  Future<void> reconnect(HubEndpoint endpoint) async {
+  /// [authToken] is read from `~/.fai/hub-auth-token` by
+  /// default — pass `null` to drop a previously-loaded token,
+  /// or omit the parameter to keep the current value.
+  Future<void> reconnect(
+    HubEndpoint endpoint, {
+    Object? authToken = _unset,
+  }) async {
     await _client.close();
-    _client = HubClient(endpoint: endpoint);
+    final token = identical(authToken, _unset)
+        ? await HubAuthToken.read()
+        : authToken as String?;
+    _client = HubClient(endpoint: endpoint, authToken: token);
     final prefs = await SharedPreferences.getInstance();
     await prefs.setString(_kHostKey, endpoint.host);
     await prefs.setInt(_kPortKey, endpoint.port);
     await prefs.setBool(_kSecureKey, endpoint.secure);
   }
 
+  /// Reload the token from disk and reconnect using the current
+  /// endpoint. Called by Settings after the operator pastes or
+  /// clears a token.
+  Future<void> reloadAuthToken() async {
+    await reconnect(_client.endpoint);
+  }
+
   static const _kThemeKey = 'theme.mode';
   static const _kLocaleKey = 'locale.code';