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/pages/approvals.dart

@@ -89,6 +89,12 @@ class _ApprovalsPageState extends State<ApprovalsPage>
         limit: 200,
         project: project,
       );
+      // Only the visible tab has a FutureBuilder listening; give the
+      // hidden tab's future a silent listener so a load failure there
+      // never surfaces as an uncaught async error (the FutureBuilder
+      // that attaches on tab switch still receives the error).
+      _pendingFuture.then((_) {}, onError: (_) {});
+      _historyFuture.then((_) {}, onError: (_) {});
     });
   }
 
@@ -331,7 +337,6 @@ class _PendingList extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    final theme = Theme.of(context);
     final l = AppLocalizations.of(context)!;
     return FutureBuilder<List<ApprovalRecord>>(
       future: future,
@@ -340,16 +345,7 @@ class _PendingList extends StatelessWidget {
           return const Center(child: CircularProgressIndicator());
         }
         if (snapshot.hasError) {
-          return ChainEmptyState(
-            icon: Icons.cloud_off_outlined,
-            iconColor: theme.colorScheme.error,
-            title: l.hubUnreachable,
-            hint: l.hubUnreachableHint,
-            action: FilledButton.tonal(
-              onPressed: onRetry,
-              child: Text(l.buttonRetry),
-            ),
-          );
+          return HubLoadErrorView(error: snapshot.error!, onRetry: onRetry);
         }
         final pending = snapshot.data ?? [];
         if (pending.isEmpty) {
@@ -513,7 +509,6 @@ class _HistoryList extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    final theme = Theme.of(context);
     final l = AppLocalizations.of(context)!;
     return FutureBuilder<List<ApprovalRecord>>(
       future: future,
@@ -522,16 +517,7 @@ class _HistoryList extends StatelessWidget {
           return const Center(child: CircularProgressIndicator());
         }
         if (snapshot.hasError) {
-          return ChainEmptyState(
-            icon: Icons.cloud_off_outlined,
-            iconColor: theme.colorScheme.error,
-            title: l.hubUnreachable,
-            hint: l.hubUnreachableHint,
-            action: FilledButton.tonal(
-              onPressed: onRetry,
-              child: Text(l.buttonRetry),
-            ),
-          );
+          return HubLoadErrorView(error: snapshot.error!, onRetry: onRetry);
         }
         final decided = snapshot.data ?? [];
         if (decided.isEmpty) {