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/audit.dart

@@ -6,6 +6,7 @@ import 'package:file_picker/file_picker.dart';
 import 'package:flutter/material.dart';
 
 import '../data/error_presentation.dart';
+import '../data/friendly_error.dart';
 import '../data/hub.dart';
 import '../data/workspace.dart';
 import '../l10n/app_localizations.dart';
@@ -69,7 +70,7 @@ class _AuditPageState extends State<AuditPage> {
   String _typeFilter = 'all';
   String _search = '';
   List<AuditEvent> _events = const [];
-  String? _error;
+  Object? _error;
   bool _initialLoaded = false;
   Timer? _poller;
 
@@ -274,7 +275,7 @@ class _AuditPageState extends State<AuditPage> {
     } catch (e) {
       if (!mounted) return;
       setState(() {
-        _error = e.toString();
+        _error = e;
         _initialLoaded = true;
       });
     }
@@ -363,7 +364,18 @@ class _AuditPageState extends State<AuditPage> {
       ),
       body: Column(
         children: [
-          _LiveStatusBar(eventCount: filtered.length, error: _error),
+          // The status bar shows the CLASSIFIED one-liner, never the
+          // raw thrown object — walls of gRPC text belong behind the
+          // error state's detail expander (state-matrix invariant).
+          _LiveStatusBar(
+            eventCount: filtered.length,
+            error: _error == null
+                ? null
+                : friendlyError(
+                    _error!,
+                    AppLocalizations.of(context)!,
+                  ).headline,
+          ),
           Padding(
             padding: const EdgeInsets.fromLTRB(
               ChainSpace.xl,
@@ -388,12 +400,7 @@ class _AuditPageState extends State<AuditPage> {
             child: !_initialLoaded
                 ? const Center(child: CircularProgressIndicator())
                 : _error != null && _events.isEmpty
-                ? ChainEmptyState(
-                    icon: Icons.cloud_off_outlined,
-                    iconColor: theme.colorScheme.error,
-                    title: AppLocalizations.of(context)!.hubUnreachable,
-                    hint: AppLocalizations.of(context)!.hubUnreachableHint,
-                  )
+                ? HubLoadErrorView(error: _error!, onRetry: _refresh)
                 : filtered.isEmpty
                 ? ChainEmptyState(
                     icon: Icons.timeline_outlined,