feat(app,learn): shared spaces' members on the device

Step 4 (e) of ADR 0044, its first part, for every app's screens.

- halkyon_app: `SharedSpaceGateway` on the module's `sharedSpace`
  endpoint (the spaces, their members and roles, inviting, handing on,
  leaving), refusals as `SpaceRefused`; `SharedKeys.trustIn` gives
  each member's safety code and whether it was compared or their keys
  changed, `markCompared`, and `trustNewKeys`, which pins the keys
  whose code was compared and never those the instance hands next.
  `FakeSharedSpaceServer` keeps the module's rules for tests, in step
  with `FakeSharedKeysServer`, which now knows members' handles.
- Learn's sharing makes its space through it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_app/lib/testing.dart

@@ -23,6 +23,7 @@ export 'src/testing/fake_password_gateway.dart';
 export 'src/testing/fake_public_profile_gateway.dart';
 export 'src/testing/fake_screen_guard.dart';
 export 'src/testing/fake_shared_keys_gateway.dart';
+export 'src/testing/fake_shared_space_gateway.dart';
 export 'src/testing/fake_simplelogin_gateway.dart';
 export 'src/testing/fake_speech_input.dart';
 export 'src/testing/memory_account_key_store.dart';