feat(app): signing in with SimpleLogin, its sheet and the start screen's offer (ADR 0020)

Where the instance offers SimpleLogin, the start screen offers it first,
saying that the instance keeps only the SimpleLogin ID, and opens a sheet
that waits for SimpleLogin's page in the browser (Cancel ends the wait),
then: for someone new, the choice between creating an account (the
suggested handle or another, checked with the instance, the password
twice, the recovery phrase shown once and three of its words typed back,
nothing reaching the instance before) and linking theirs again with its
handle and recovery phrase; for an account, its password opening its
keys; for an account of before, the keys made with the handle it lacks.
SimpleLoginCubit holds the steps, each refusal shown where it is mended
(a handle's on the handle, a sign-in to begin again on the browser's),
and closing the sheet stops waiting on the loopback.

The suggested handle is never checked: the instance suggests none that
anybody has, and every answer about a handle counts against its limit
(D5). The fake's handle rule is now the module's.

Learn and Todo build it with SimpleLogin's browser (halkyon_oidc_flutter,
now a direct dependency of both, with their names on the loopback's
page), their schemes and their loopback ports (43823, 43824), as the
instance's forwarding page has them. Their tests offer no SimpleLogin;
halkyon_app's test the sheet.

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

apps/learn/halkyon_learn_flutter/test/support/test_dependencies.dart

@@ -179,17 +179,30 @@ Dependencies testDependencies({
   final device = deviceDictionaries ?? noDeviceDictionaries();
   final auth = authGateway ?? FakeAuthGateway();
   final addresses = signInAddresses ?? MemorySignInAddresses();
+  final crypto = FakeAccountCrypto();
+  final keys = accountKeys ?? MemoryAccountKeyStore();
   return Dependencies(
     serverName: 'learn.example.org',
     auth: AuthRepository(auth, addresses),
     signedOut: signedOut ?? MemorySignedOutMark(),
     passwords: PasswordRepository(
       passwordGateway ?? FakePasswordGateway(auth),
-      FakeAccountCrypto(),
-      accountKeys ?? MemoryAccountKeyStore(),
+      crypto,
+      keys,
       auth,
       addresses,
     ),
+    // Offered by no instance in Learn's tests: halkyon_app's tests test its
+    // sheet.
+    simpleLogin: SimpleLoginRepository(
+      FakeSimpleLoginGateway(auth)..isOffered = false,
+      FakeOidcBrowser(),
+      crypto,
+      keys,
+      auth,
+      callbackScheme: 'app.halkyon.learn',
+      loopbackPort: 43823,
+    ),
     sourceCodeUrl:
         sourceCodeUrl ?? () async => 'https://example.org/halkyon-learn',
     dictionaries: DictionaryRepository(