fix(shell): auth-rejected hub no longer reported as unreachable

The sustained-failure banner treated every failed health poll as
'can't reach the hub'. With token auth active, a wrong or rotated
token gets UNAUTHENTICATED from a perfectly reachable hub — the
old wording sent the operator to fix the endpoint. The shell now
uses the SDK's probe() and, on auth rejection, switches the banner
to 'rejected the sign-in — check the access token' (key-off icon,
DE+EN). Two widget tests pin the wording per failure kind and the
banner clearing once the probe turns serving.

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

lib/data/hub.dart

@@ -212,6 +212,10 @@ class HubService {
 
   Future<bool> healthy() => _client.healthy();
 
+  /// Health probe that keeps the failure kind: an auth-rejected
+  /// hub is up and needs a token fix, not an endpoint fix.
+  Future<HubProbeResult> probeHealth() => _client.probe();
+
   /// Configured module stores (+ the bundled seed) for the store manager.
   Future<List<StoreSource>> listStores() => _client.listStores();