feat: wire chain_client_sdk gRPC client + HubRepository scaffold

Adds a real Ch∆In hub backend to the Flutter client as a
sibling-path dependency on chain_client_sdk_dart, alongside the
existing MockRepository (which stays the default until the
flow.completed-to-Evaluation pipeline lands in Phase 0 week 1).

Wiring

  HubSettings (lib/data/hub_endpoint.dart) — persists host /
  port / secure / token / useHub flags via SharedPreferences
  under the Studio-compatible hub.* keys. Distinct from the
  SDK's chain.HubEndpoint, which is a transport-level value
  type — HubSettings adapts in that direction.

  HubRepository (lib/data/hub_repository.dart) — implements
  EvaluationRepository on top of HubClient. connect() runs a
  4-second healthy() probe; on failure returns a repository
  with isHealthy=false rather than throwing, so the UI can
  surface "hub-down" without an exception-handling round-trip.
  list() / byEli() return empty data in Phase 0 because no
  hub-side flow yet produces Evaluation objects in the
  lawheatmap-specific shape.

  main.dart — loads HubSettings on startup; if useHub=false
  starts with MockRepository (fast offline default), otherwise
  probes the hub and falls back to MockRepository on probe
  failure. No mid-session swap to keep the Phase 0 wiring
  simple; a settings save instructs the user to restart.

  HubStatusPage — read-only summary card at the top, editable
  endpoint form, "Verbindung testen" probe button, "Speichern"
  persist button. Live-flow runbook below describes the
  chain serve / install / run / approve sequence so a user
  can wire the real flow themselves once Phase 1 lands.

flutter analyze: clean. flutter test: 2/2 passing.
Smoke-launch on macOS: app boots, Dart VM service comes up,
no runtime errors in mock mode.

Signed-off-by: flemming-it <sf@flemming.it>

app/lib/data/hub_repository.dart

@@ -0,0 +1,74 @@
+import 'package:chain_client_sdk/chain_client_sdk.dart' as chain;
+
+import 'hub_endpoint.dart';
+import 'models.dart';
+import 'repository.dart';
+
+/// Live `EvaluationRepository` backed by a Ch∆In hub over gRPC.
+///
+/// Phase 0 scope: open a HubClient, run a healthy() probe, and
+/// surface the connection state to the UI. The list() / byEli()
+/// methods return empty data because no flow on the hub yet
+/// produces Evaluation objects in the lawheatmap-specific shape —
+/// that pipeline lands in Phase 0 week 1 once a chain run wraps
+/// flows/durchstich-gewo-14.yaml output into the Evaluation
+/// schema.
+///
+/// Until then [MockRepository] stays the default. Pick this
+/// repository explicitly via the Hub-settings page; on first
+/// healthy() failure the app falls back to MockRepository so the
+/// UI keeps working.
+class HubRepository implements EvaluationRepository {
+  HubRepository._(this._client, this.settings, this._healthy);
+
+  final chain.HubClient _client;
+  final HubSettings settings;
+  final bool _healthy;
+
+  /// Try to connect. Returns a repository with `isHealthy=false`
+  /// if the probe times out or the hub is unreachable; the
+  /// caller decides whether to use it or fall back to
+  /// MockRepository.
+  static Future<HubRepository> connect(HubSettings settings) async {
+    final endpoint = chain.HubEndpoint(
+      host: settings.host,
+      port: settings.port,
+      secure: settings.secure,
+    );
+    final client = chain.HubClient(
+      endpoint: endpoint,
+      authToken: settings.authToken,
+    );
+    final ok = await client.healthy().timeout(
+          const Duration(seconds: 4),
+          onTimeout: () => false,
+        );
+    return HubRepository._(client, settings, ok);
+  }
+
+  bool get isHealthy => _healthy;
+
+  /// Underlying SDK client — exposed so the hub-status page can
+  /// list capabilities, query the audit log, and verify the
+  /// event-chain without leaking gRPC types to other pages.
+  chain.HubClient get client => _client;
+
+  @override
+  Future<List<Evaluation>> list() async {
+    // Phase 0 placeholder. The pipeline that turns
+    // flow.completed events into Evaluation objects lives in the
+    // surrounding flow (flows/durchstich-gewo-14.yaml) and lands
+    // in Phase 0 week 1.
+    return const [];
+  }
+
+  @override
+  Future<Evaluation?> byEli(String eli) async => null;
+
+  @override
+  String get mode => _healthy ? 'hub-live' : 'hub-down';
+
+  @override
+  String get hubLabel => '${settings.url}'
+      '${_healthy ? '' : ' — Verbindung gestört'}';
+}