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_endpoint.dart

@@ -0,0 +1,83 @@
+import 'package:shared_preferences/shared_preferences.dart';
+
+/// Persisted user settings for the Ch∆In hub connection.
+///
+/// Stored under four SharedPreferences keys matching the Studio
+/// convention (`hub.host`, `hub.port`, `hub.secure`, `hub.token`)
+/// so a hub once configured stays configured across app launches.
+/// Distinct from the SDK's `chain.HubEndpoint`, which is a
+/// transport-level value type — [HubSettings.toEndpoint] adapts
+/// in that direction.
+class HubSettings {
+  const HubSettings({
+    required this.host,
+    required this.port,
+    required this.secure,
+    this.authToken,
+    this.useHub = false,
+  });
+
+  final String host;
+  final int port;
+  final bool secure;
+
+  /// Bearer token; read from `~/.chain/hub-auth-token` by the
+  /// hub-status page in Phase 1. Optional in Phase 0 because the
+  /// default `chain serve` runs without auth.
+  final String? authToken;
+
+  /// User intent: try to connect to the hub on startup. False
+  /// keeps the app in MockRepository mode regardless of host/port.
+  final bool useHub;
+
+  static const defaultHost = '127.0.0.1';
+  static const defaultPort = 50051;
+
+  String get scheme => secure ? 'https' : 'http';
+  String get url => '$scheme://$host:$port';
+
+  static const _kHost = 'hub.host';
+  static const _kPort = 'hub.port';
+  static const _kSecure = 'hub.secure';
+  static const _kToken = 'hub.token';
+  static const _kUseHub = 'hub.use';
+
+  static Future<HubSettings> load() async {
+    final prefs = await SharedPreferences.getInstance();
+    return HubSettings(
+      host: prefs.getString(_kHost) ?? defaultHost,
+      port: prefs.getInt(_kPort) ?? defaultPort,
+      secure: prefs.getBool(_kSecure) ?? false,
+      authToken: prefs.getString(_kToken),
+      useHub: prefs.getBool(_kUseHub) ?? false,
+    );
+  }
+
+  Future<void> save() async {
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setString(_kHost, host);
+    await prefs.setInt(_kPort, port);
+    await prefs.setBool(_kSecure, secure);
+    await prefs.setBool(_kUseHub, useHub);
+    if (authToken != null) {
+      await prefs.setString(_kToken, authToken!);
+    } else {
+      await prefs.remove(_kToken);
+    }
+  }
+
+  HubSettings copyWith({
+    String? host,
+    int? port,
+    bool? secure,
+    String? authToken,
+    bool? useHub,
+  }) =>
+      HubSettings(
+        host: host ?? this.host,
+        port: port ?? this.port,
+        secure: secure ?? this.secure,
+        authToken: authToken ?? this.authToken,
+        useHub: useHub ?? this.useHub,
+      );
+}