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>

test/responsive_test.dart

@@ -13,6 +13,8 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:chain_studio/data/hub.dart';
 import 'package:chain_studio/main.dart';
 
+import 'support/fake_hub.dart';
+
 const _destinations = <String>[
   'welcome',
   'store',
@@ -35,6 +37,9 @@ void main() {
   for (final entry in _sizes.entries) {
     testWidgets('all pages lay out without overflow — ${entry.key}',
         (tester) async {
+      // Hermetic: pages render against the scriptable fake, never a
+      // hub that happens to listen on the operator's machine.
+      installFakeHub();
       tester.view.physicalSize = entry.value;
       tester.view.devicePixelRatio = 1.0;
       addTearDown(tester.view.reset);