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>

packages/halkyon_app/lib/l10n/halkyon_app_localizations_en.dart

@@ -894,6 +894,144 @@ class HalkyonAppLocalizationsEn extends HalkyonAppLocalizations {
   @override
   String get passwordSignedInBefore =>
       'Only a device signed in since then can dismiss this warning: sign in again on this one, or change your password.';
+
+  @override
+  String get simpleLoginContinue => 'Continue with SimpleLogin';
+
+  @override
+  String simpleLoginNote(String instance) {
+    return 'SimpleLogin is Proton\'s sign-in, and a Proton account works there too. It learns that you use $instance, which keeps only your SimpleLogin ID: your name suggests a handle once, and nothing else is kept.';
+  }
+
+  @override
+  String get simpleLoginTitle => 'Sign in with SimpleLogin';
+
+  @override
+  String get simpleLoginAddTitle => 'Add SimpleLogin';
+
+  @override
+  String get simpleLoginInBrowser => 'Continue in your browser.';
+
+  @override
+  String get simpleLoginNewcomerTitle => 'New here?';
+
+  @override
+  String simpleLoginNewcomerIntro(String instance) {
+    return 'Your SimpleLogin account has no account on $instance yet.';
+  }
+
+  @override
+  String get simpleLoginCreateHint =>
+      'Choose a handle and a password; you get a recovery phrase.';
+
+  @override
+  String get simpleLoginHaveOne => 'I already have one';
+
+  @override
+  String get simpleLoginHaveOneHint =>
+      'Link it again with your handle and your recovery phrase.';
+
+  @override
+  String get simpleLoginHandleIntro =>
+      'Others find you by your handle. It stays private until you publish something or fill in your profile, and can change until your first publish.';
+
+  @override
+  String get simpleLoginHandle => 'Handle';
+
+  @override
+  String simpleLoginHandleFree(String handle) {
+    return '@$handle is free';
+  }
+
+  @override
+  String get simpleLoginPasswordIntro =>
+      'Choose a password: you type it when you sign in on another device. It never leaves this device: the instance can\'t read it, and can\'t reset it without what it protects being lost.';
+
+  @override
+  String get simpleLoginCreate => 'Create the account';
+
+  @override
+  String get simpleLoginUnlockTitle => 'Your password';
+
+  @override
+  String simpleLoginUnlockIntroAs(String handle) {
+    return 'Signed in with SimpleLogin as @$handle. Your password opens your keys on this device.';
+  }
+
+  @override
+  String get simpleLoginUnlockIntro =>
+      'Signed in with SimpleLogin. Your password opens your keys on this device.';
+
+  @override
+  String get simpleLoginOpen => 'Open';
+
+  @override
+  String get simpleLoginMendTitle => 'Link your account';
+
+  @override
+  String get simpleLoginMendIntro =>
+      'Your SimpleLogin account isn\'t the one your account knows any more. Link them again with your handle and your recovery phrase: your other devices are then signed out.';
+
+  @override
+  String get simpleLoginMend => 'Link';
+
+  @override
+  String get simpleLoginWrongPassword => 'The password isn\'t right.';
+
+  @override
+  String get simpleLoginMendDoesNotFit =>
+      'The handle or the recovery phrase isn\'t right.';
+
+  @override
+  String get simpleLoginUnavailable =>
+      'This instance doesn\'t offer sign-in with SimpleLogin.';
+
+  @override
+  String get simpleLoginDenied =>
+      'SimpleLogin\'s sign-in was declined, so nothing was shared.';
+
+  @override
+  String get simpleLoginNoBrowser =>
+      'There\'s no browser to open SimpleLogin\'s page in.';
+
+  @override
+  String get simpleLoginStartAgain =>
+      'The sign-in took too long, or another one took its place. Begin it again.';
+
+  @override
+  String get simpleLoginRejected =>
+      'SimpleLogin didn\'t confirm the sign-in. Begin it again.';
+
+  @override
+  String get simpleLoginProviderUnavailable =>
+      'The instance can\'t reach SimpleLogin right now. Try again later.';
+
+  @override
+  String get simpleLoginTakenByAnotherPerson =>
+      'This SimpleLogin account belongs to another account here.';
+
+  @override
+  String get simpleLoginOtherIdentityLinked =>
+      'Your account has another SimpleLogin account already.';
+
+  @override
+  String get simpleLoginTooManyAttempts =>
+      'Too many sign-ins began from this network. Try again in an hour.';
+
+  @override
+  String get simpleLoginHandleInvalid =>
+      'A handle is 1 to 30 letters (a to z), digits or underscores.';
+
+  @override
+  String get simpleLoginHandleReserved =>
+      'This handle is kept for the instance.';
+
+  @override
+  String get simpleLoginHandleTaken => 'Someone else has this handle.';
+
+  @override
+  String get simpleLoginTooManyHandleChecks =>
+      'Too many handles were tried. Try again in an hour.';
 }
 
 /// The translations for English, as used in the United Kingdom (`en_GB`).