feat(oidc): halkyon_oidc_flutter, the device's half of signing in with OpenID Connect (ADR 0023)

OidcBrowser opens the page of an attempt the server began (OidcStart),
waits until the browser comes back by an OidcReturn (the app's scheme,
a verified https address, or the loopback) and returns the provider's
code (OidcCode), its refusal (OidcDenied), or nothing when the person
closed the browser. It checks the returned state against the attempt's,
as the server never compares it; a wrong state, neither one code nor
one refusal, no browser or a failed way back are OidcCallbackExceptions.

halkyon_app's LoopbackWebAuth moves in with its tests, relicensed from
AGPL v3 to BSD-3-Clause by its author, the developer; GitHub's sign-in
keeps using it from there until it ends. The package is BSD-3-Clause,
publish_to: none, as the module (ADR 0023, D2, D3).

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/pubspec.yaml

@@ -29,8 +29,9 @@ dependencies:
   # The account's private keys, once unlocked, in the system's keyring
   # (ADR 0017), as Serverpod keeps sessions.
   flutter_secure_storage: ^10.0.0
-  # LoopbackWebAuth takes the place of the package's own browser half on
-  # Linux and Windows.
+  # GitHub's sign-in goes through the package's front, so LoopbackWebAuth
+  # (halkyon_oidc_flutter) takes the place of its browser half on Linux and
+  # Windows.
   flutter_web_auth_2_platform_interface: ^5.0.0
   halkyon_base_client:
     path: ../halkyon_base_client
@@ -40,6 +41,8 @@ dependencies:
   # hook compiles, so every build of an app needs a C compiler and make.
   halkyon_keys:
     path: ../halkyon_keys
+  halkyon_oidc_flutter:
+    path: ../halkyon_oidc_flutter
   halkyon_ui:
     path: ../halkyon_ui
   # A client of halkyon_base alone can be given a fake one in tests.
@@ -63,7 +66,6 @@ dependencies:
   # opening it on each isolate the account's keys are worked on.
   sodium: 4.1.0+1
   url_launcher: ^6.3.2
-  window_to_front: ^0.0.4
 
 dev_dependencies:
   fake_async: ^1.3.3