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/todo/AGENTS.md

@@ -4,6 +4,6 @@ What holds for every app and package of this repository is in the [root AGENTS.m
 
 - **A shell on the shared packages** (decided 2026-10-19): signing in, the instances, the account and the look work; nothing of a person's is kept or synced yet, which waits for Todo's first model (the developer decides what it keeps and syncs). Packages: `halkyon_todo_server`, `halkyon_todo_client` (generated, never edited), `halkyon_todo_flutter`; app ID `app.halkyon.todo` on every platform; no web build. Licences as Learn's (server SSPL, client and app AGPL; `LICENSE.md`).
 - **The server** includes `halkyon_base` (ADR 0014) and configures it in `server.dart` (`todoBase()`, `lib/src/todo_base.dart`): its basics say it serves `todo` (never change it once apps use it), sign-in mails name Halkyon Todo, the source is this repository, `minimumAppBuild` 1, no data of its own to delete with a person yet (add a `deletePersonData` hook with Todo's first table). Authentication through the module's builders: anonymous, GitHub (with the instance's own OAuth app) and email codes, server-side sessions. **Accounts** (ADR 0016): Todo holds its own register when it runs alone, and is a member of the instance's register (Learn's server on learn.halkyon.eu) where the operator sets `ACCOUNT_REGISTER_URL` (`todoBase(accounts: AccountsMode.fromEnvironment())`) with the instance's shared `INSTANCE_SECRET` (`instanceSecret`); then sign-in, handles and profiles are the register's, the email settings are the register's alone (GitHub's are Todo's own OAuth app's), and an identified person's deletion goes to the register first. `addInstanceRoutes` serves the calls between the services on its web server, and the server doesn't start without the secret where it needs it. `schedulePruning` (`lib/src/pruning.dart`) keeps the module's pruning scheduled, one entry each however often the server starts: anonymous people daily, sign-up attempts and codes hourly, as the register the deletions its members haven't confirmed hourly, as a member the people the register no longer knows daily (each a no-op in the other mode). Session logs are never persisted, in any run mode. Migrations (2026-10-19): the initial one (Serverpod's tables, its auth modules' and the module's `email_identity`, `email_code_request`), `profiles` (`public_profile`) and `deletion-notices` (`account_deletion_notice`), nothing of Todo's own. Tests (`test/integration/todo_server_test.dart`) call the module's endpoints through the server's own dispatch (`halkyon_base.<endpoint>`), since the generated test tools expose the project's endpoints alone.
-- **The app** is halkyon_app's `InstanceGate` and `SessionGate` with Todo's frame (`todoFrame`: "Todo" under "Halkyon", its icon, `todo.example.org`, its own three sentences) and `ServerpodInstanceChecker(app: 'todo')`; `TodoDependencies` (`AppDependencies<TodoPerson>`) on one instance, `TodoPerson` (empty) per person; a person's app with an empty home and the account sheet: halkyon_app's `AccountTiles` (whose account on which instance, Switch account, Sign out, which asks first for an anonymous account as it deletes it, Delete account, which asks first) in Todo's words, and halkyon_app's About (`showAppAbout`: the source of what the instance runs, from `TodoDependencies.sourceCodeUrl`, and the licence list). Texts through gen-l10n in British English (the template), German (informal du), an empty `en_US` and `en_GB` (`test/l10n/arb_files_test.dart` keeps them in step). The look follows the system's theme mode and language until Todo has settings. Countries in where an instance keeps its data are named by halkyon_app's CLDR names (`AssetCountryNames`), its language picked by halkyon_app's `resolveAppLocale`. Widget tests start Todo's whole tree with halkyon_app's fakes (`test/app/todo_app_test.dart`).
+- **The app** is halkyon_app's `InstanceGate` and `SessionGate` with Todo's frame (`todoFrame`: "Todo" under "Halkyon", its icon, `todo.example.org`, its own sentence) and `ServerpodInstanceChecker(app: 'todo')`; `TodoDependencies` (`AppDependencies<TodoPerson>`) on one instance, `TodoPerson` (empty) per person; a person's app with an empty home and the account sheet: halkyon_app's `AccountTiles` (whose account on which instance, Switch account, Sign out, which asks first for an anonymous account as it deletes it, Delete account, which asks first) in Todo's words, and halkyon_app's About (`showAppAbout`: the source of what the instance runs, from `TodoDependencies.sourceCodeUrl`, and the licence list). Texts through gen-l10n in British English (the template), German (informal du), an empty `en_US` and `en_GB` (`test/l10n/arb_files_test.dart` keeps them in step). The look follows the system's theme mode and language until Todo has settings. Countries in where an instance keeps its data are named by halkyon_app's CLDR names (`AssetCountryNames`), its language picked by halkyon_app's `resolveAppLocale`. Widget tests start Todo's whole tree with halkyon_app's fakes (`test/app/todo_app_test.dart`).
 - **The icon** is Lucide's `list-checks` (lucide-static 1.50.0) in the brand's blues on white: `apps/todo/tool/app_icons.py` (run from `apps/todo`) fetches it, draws its strokes with Pillow (no SVG renderer needed; other path commands fail loudly), writes `assets/icon/halkyon_todo_icon.png` and Lucide's ISC licence beside it, then every platform's launcher icon through flutter_launcher_icons as Learn's tool does. `registerLucideLicense` puts the notice in the licence list.
 - **`serverpod start`** runs in `apps/todo/halkyon_todo_server` (it launches the app from `lib/driver.dart`); `.mcp.json`'s `--server-dir` names it when a session works on Todo. Its tests need throwaway passwords in its git-ignored `config/passwords.yaml` (`database`, `serviceSecret`, `serverSideSessionKeyHashPepper`, `emailSignInPepper`) or from the environment, as CI gives them. Learn's and Todo's development servers use the same ports: run one at a time.