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/widgets/hub_load_error_view.dart

@@ -0,0 +1,80 @@
+// HubLoadErrorView — the one way a page presents "my content
+// could not be loaded". Pages used to open-code a "hub not
+// reachable" empty state for ANY failure, contradicting the
+// sidebar's green connected dot whenever the hub answered but the
+// RPC failed (an older hub without the RPC, a feature gate, a real
+// bug). This widget routes the failure through the shared
+// classification instead:
+//
+//   * unreachable  -> the honest "hub not reachable" state
+//   * unsupported  -> "this view needs a newer hub version"
+//   * other        -> "could not load" + the friendly error in a
+//                     copyable detail box (hard rule: errors are
+//                     always copyable)
+//
+// The state-matrix suite pins the behaviour across every page.
+
+import 'package:flutter/material.dart';
+
+import '../data/friendly_error.dart';
+import '../l10n/app_localizations.dart';
+import 'chain_empty_state.dart';
+import 'chain_error_box.dart';
+
+class HubLoadErrorView extends StatelessWidget {
+  final Object error;
+
+  /// Re-runs the page's load. Rendered on the unreachable and
+  /// load-failed states when provided.
+  final VoidCallback? onRetry;
+
+  const HubLoadErrorView({super.key, required this.error, this.onRetry});
+
+  @override
+  Widget build(BuildContext context) {
+    final theme = Theme.of(context);
+    final l = AppLocalizations.of(context)!;
+    final retry = onRetry == null
+        ? null
+        : FilledButton.tonal(
+            onPressed: onRetry,
+            child: Text(l.buttonRetry),
+          );
+    switch (classifyHubLoadError(error)) {
+      case HubLoadIssue.unreachable:
+        return ChainEmptyState(
+          icon: Icons.cloud_off_outlined,
+          iconColor: theme.colorScheme.error,
+          title: l.hubUnreachable,
+          hint: l.hubUnreachableHint,
+          action: retry,
+        );
+      case HubLoadIssue.unsupported:
+        return ChainEmptyState(
+          icon: Icons.system_update_alt_outlined,
+          title: l.hubViewTooOldTitle,
+          hint: l.hubViewTooOldHint,
+          action: retry,
+        );
+      case HubLoadIssue.other:
+        return ChainEmptyState(
+          icon: Icons.error_outline,
+          iconColor: theme.colorScheme.error,
+          title: l.hubViewLoadFailedTitle,
+          action: Column(
+            mainAxisSize: MainAxisSize.min,
+            children: [
+              ConstrainedBox(
+                constraints: const BoxConstraints(maxWidth: 420),
+                child: ChainErrorBox(error: error, isError: true),
+              ),
+              if (retry != null) ...[
+                const SizedBox(height: 12),
+                retry,
+              ],
+            ],
+          ),
+        );
+    }
+  }
+}