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/pubspec.yaml

@@ -81,6 +81,11 @@ dependencies:
     path: ../../../packages/halkyon_core
   halkyon_learn_client:
     path: ../halkyon_learn_client
+  # The device's half of signing in with OpenID Connect (ADR 0023): the
+  # browser that SimpleLogin's sign-in opens, and on Linux and Windows the
+  # loopback it comes back to, which the app builds with its name.
+  halkyon_oidc_flutter:
+    path: ../../../packages/halkyon_oidc_flutter
   # Halkyon's look, shared by its apps (a shared package): the theme, the bundled
   # Noto Sans, window sizes and the widgets every screen is built of.
   halkyon_ui: