feat(test): hermetic hub fake + state-matrix sweep across all pages

The widget suites used to talk to whatever listens on the real
endpoint — results depended on the operator's machine (a running
hub fed real data into a11y/responsive runs and its gRPC channel
timers caused the historic flake). Hardening round:

- HubService.instance is now injectable (debugSetInstance);
  FakeHubService (test/support/fake_hub.dart) answers every member
  the pages touch with healthy-empty defaults and scripts per-RPC
  failures (UNIMPLEMENTED / UNAVAILABLE / detached gate) through a
  GrpcError-shaped fake. Unimplemented members are recorded and
  fail the sweep with the exact list.
- state_matrix_test.dart pins the app-wide invariants for every
  sidebar page x hub condition: healthy => no unreachable claims
  and no raw error text; hub gone => honest unreachable states;
  UNIMPLEMENTED => never 'not reachable' while the sidebar shows
  connected; detached gate => plain-language feature-off state.
- a11y + responsive sweeps now inject the fake (hermetic); the
  6-minute idle-timer drain workaround is gone with the cause.

Real bugs the new sweep caught immediately:
- every data page (store, doctor, audit, approvals, federation)
  folded ANY load failure into 'hub not reachable' — the runs-page
  bug class; they now share HubLoadErrorView, which classifies
  into unreachable / needs-newer-hub / load-failed-with-copyable-
  detail (new generic DE+EN strings)
- the approvals page's hidden tab had no future listener: a load
  failure there surfaced as an uncaught async error
- the audit status bar rendered the raw gRPC error wall verbatim;
  it now shows the classified friendly headline (still selectable)

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

lib/data/hub.dart

@@ -20,7 +20,17 @@ export 'flow_output.dart';
 
 class HubService {
   HubService._();
-  static final HubService instance = HubService._();
+  static HubService instance = HubService._();
+
+  /// Test hook: replace the app-wide instance with a scriptable
+  /// fake so widget suites are hermetic — they must never depend
+  /// on whether a real hub happens to listen on the operator's
+  /// machine (the state-matrix suite injects per-RPC behaviours
+  /// through this). Pass null to restore a real instance.
+  @visibleForTesting
+  static void debugSetInstance(HubService? replacement) {
+    instance = replacement ?? HubService._();
+  }
 
   HubClient _client = HubClient();