feat(app)!: no anonymous accounts on the device (ADR 0020, D4, D25)

Every account has a way in, so the apps make no anonymous account any
more. The start screen shows whenever nobody is signed in, a first
start included, and offers SimpleLogin, a password and GitHub for those
who have it, with no start without an account. An instance out of reach
is said so, with Try again, and one offering no way to create an account
too. An anonymous account's session left from before is deleted as the
app starts (D25), falling back to signing out on the device.

What only anonymous accounts needed goes: signing in anonymously, the
signed-out mark, adding GitHub to an account and switching to a GitHub
account from the Profile tab (with its Cancel, the only one), an
anonymous device's own keys, the warning that signing out deletes the
account, and the texts about keeping progress. Settings says on which
instance the account is signed in. Learn's publishing, reports and
suggestions and its learner count no longer ask whether the person has
an identity: everyone signed in has one.

The server still offers anonymous sign-in until the next commit, so
older apps keep working until v0.20.0 raises the minimum build.

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/pump_app.dart

@@ -9,8 +9,8 @@ import 'package:halkyon_learn_flutter/src/feature/studyset/widget/studyset_parts
 import 'package:material_ui/material_ui.dart';
 
 /// Pumps the whole app as a person starts it, on [dependencies] (usually
-/// `testDependencies(...)`): signed in, anonymously unless the auth fake says
-/// otherwise, and opened at [location]. Settles, so that the person's data is
+/// `testDependencies(...)`): signed in as the auth fake's person unless it
+/// says otherwise, and opened at [location]. Settles, so that the person's data is
 /// open and the first screen shows; the app's spinner animates without end,
 /// so settling needs the fakes to have answered.
 ///

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

@@ -124,7 +124,6 @@ import 'memory_studyset_tab_store.dart';
 /// the person asks; never on timers, which a widget test would find pending.
 Dependencies testDependencies({
   AuthGateway? authGateway,
-  SignedOutMark? signedOut,
   SignInAddresses? signInAddresses,
   StudysetGateway? studysetGateway,
   CardGateway? cardGateway,
@@ -186,7 +185,6 @@ Dependencies testDependencies({
   return Dependencies(
     serverName: 'learn.example.org',
     auth: AuthRepository(auth, addresses),
-    signedOut: signedOut ?? MemorySignedOutMark(),
     passwords: PasswordRepository(
       passwordGateway ?? FakePasswordGateway(auth),
       crypto,