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/pubspec.lock

@@ -17,6 +17,13 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "2.1.2"
+  chain_client_sdk:
+    dependency: "direct main"
+    description:
+      path: "../../fai_chain_client_sdk_dart"
+      relative: true
+    source: path
+    version: "0.18.0"
   characters:
     dependency: transitive
     description:
@@ -41,6 +48,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.19.1"
+  crypto:
+    dependency: transitive
+    description:
+      name: crypto
+      sha256: c8ea0233063ba03258fbcf2ca4d6dadfefe14f02fab57702265467a19f27fadf
+      url: "https://pub.dev"
+    source: hosted
+    version: "3.0.7"
   fake_async:
     dependency: transitive
     description:
@@ -65,6 +80,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "7.0.1"
+  fixnum:
+    dependency: transitive
+    description:
+      name: fixnum
+      sha256: b6dc7065e46c974bc7c5f143080a6764ec7a4be6da1285ececdc37be96de53be
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.1.1"
   flutter:
     dependency: "direct main"
     description: flutter
@@ -93,6 +116,62 @@ packages:
     description: flutter
     source: sdk
     version: "0.0.0"
+  google_cloud:
+    dependency: transitive
+    description:
+      name: google_cloud
+      sha256: b385e20726ef5315d302c5933bfb728103116c5be2d3d17094b01a82da538c1f
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.5.0"
+  google_identity_services_web:
+    dependency: transitive
+    description:
+      name: google_identity_services_web
+      sha256: "5d187c46dc59e02646e10fe82665fc3884a9b71bc1c90c2b8b749316d33ee454"
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.3.3+1"
+  googleapis_auth:
+    dependency: transitive
+    description:
+      name: googleapis_auth
+      sha256: "1417d8846663df5e7b77ca56591c5edd442c66ffc9c01ab036e138a21a148e86"
+      url: "https://pub.dev"
+    source: hosted
+    version: "2.3.2"
+  grpc:
+    dependency: transitive
+    description:
+      name: grpc
+      sha256: "86be3a7d39ad865b214a7370021ac80e68939238b507730de6d97fc662cb2723"
+      url: "https://pub.dev"
+    source: hosted
+    version: "5.1.0"
+  http:
+    dependency: transitive
+    description:
+      name: http
+      sha256: "87721a4a50b19c7f1d49001e51409bddc46303966ce89a65af4f4e6004896412"
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.6.0"
+  http2:
+    dependency: transitive
+    description:
+      name: http2
+      sha256: "382d3aefc5bd6dc68c6b892d7664f29b5beb3251611ae946a98d35158a82bbfa"
+      url: "https://pub.dev"
+    source: hosted
+    version: "2.3.1"
+  http_parser:
+    dependency: transitive
+    description:
+      name: http_parser
+      sha256: "178d74305e7866013777bab2c3d8726205dc5a4dd935297175b19a23a2e66571"
+      url: "https://pub.dev"
+    source: hosted
+    version: "4.1.2"
   intl:
     dependency: "direct main"
     description:
@@ -205,6 +284,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "2.1.8"
+  protobuf:
+    dependency: transitive
+    description:
+      name: protobuf
+      sha256: "75ec242d22e950bdcc79ee38dd520ce4ee0bc491d7fadc4ea47694604d22bf06"
+      url: "https://pub.dev"
+    source: hosted
+    version: "6.0.0"
   shared_preferences:
     dependency: "direct main"
     description:
@@ -314,6 +401,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "0.7.11"
+  typed_data:
+    dependency: transitive
+    description:
+      name: typed_data
+      sha256: f9049c039ebfeb4cf7a7104a675823cd72dba8297f264b6637062516699fa006
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.4.0"
   vector_math:
     dependency: transitive
     description: