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>

packages/halkyon_app/README.md

@@ -37,9 +37,14 @@ Written as the apps are: Dart 3.13's primary constructors, Material from
   `SubdivisionNames`).
 - **Gates** (`lib/src/app/`): `InstanceGate` opens the app's dependencies on
   the instance in use (`AppDependencies`, handed down by `AppScope`), anew on
-  a switch, the previous ones closed first; `SessionGate` keeps a person
-  signed in, signing in anonymously at a first start and asking how to start
-  once signed out (`StartScreen`); `PersonScope` opens each person's own
+  a switch, the previous ones closed first; `SessionGate` shows the app of
+  whoever is signed in and asks how to start while nobody is
+  (`StartScreen`: SimpleLogin, a password, GitHub for those who have it,
+  another instance; an instance out of reach is said so, with Try again,
+  and one offering no way to create an account too), at a first start as
+  after signing out; an anonymous account's session, left from before
+  there were none, is deleted as the app starts (ADR 0020, D25), falling
+  back to signing out on the device; `PersonScope` opens each person's own
   dependencies and closes them when the person goes. `resolveAppLocale`
   picks the app's language as every Halkyon app does: the first preferred
   language it speaks, never by country alone, British English otherwise.
@@ -48,9 +53,9 @@ Written as the apps are: Dart 3.13's primary constructors, Material from
   instance runs is (`InstanceSourceCode`, from `AppDependencies.sourceCodeUrl`;
   both licences require offering it), the app's own parts (its credits), and
   the licence list.
-- **The account** (`AccountTiles`): whose account on which instance,
-  switching to the account on another, signing out (which of an anonymous
-  account deletes it, so asks first) and deleting it (after asking), for an
+- **The account** (`AccountTiles`): on which instance the account is signed
+  in, switching to the account on another, signing out and deleting it
+  (after asking), for an
   app's settings, in the app's words where only it can say them. While one
   runs, the person's app stays and its tile shows a spinner; one that fails
   keeps them signed in and says why once, in a snack bar (in a sheet, give
@@ -58,26 +63,20 @@ Written as the apps are: Dart 3.13's primary constructors, Material from
   the same (Serverpod's `signOutDevice`), so only the device's own storage
   fails a sign-out.
 - **Signing in** (`lib/src/auth/`): `AuthRepository` on an `AuthGateway`;
-  `ServerpodAuthGateway` signs in anonymously and with GitHub (the system
-  browser and a loopback address on Linux and Windows, halkyon_oidc_flutter's
-  `LoopbackWebAuth`),
+  `ServerpodAuthGateway` signs in with GitHub those who have it, as it is
+  added to nobody any more (ADR 0020, D25; the system browser and a loopback
+  address on Linux and Windows, halkyon_oidc_flutter's `LoopbackWebAuth`),
   through the instance's `halkyon_base`, and puts a session granted elsewhere
-  in place (`useSession`); `SessionBloc`, `GitHubSignInBloc` and
-  `SignInWaysCubit`; `SignedOutMark`, `SignInAddresses`, `PersonFolders`.
-  Where the instance offers passwords, GitHub is added to nobody (ADR 0017):
-  `gitHubOffer` says so from its basics, `GitHubSignInStateReady.linking`
-  hands it to the screens, which then offer no Add, and the start screen
-  offers an account to create in place of the app's sentence on adding
-  GitHub; `gitHubProblemText` says why a GitHub sign-in was refused.
+  in place (`useSession`); `SessionBloc` and `SignInWaysCubit`;
+  `SignInAddresses`, `PersonFolders`. The start screen says why a GitHub
+  sign-in was refused, and for a GitHub account nobody here has, the way to
+  create an account in its place.
 - **Content sealed with data keys** (`lib/src/privacy/`, ADR 0019):
   `ContentCipher` (`SodiumContentCipher`, on the isolate it is used on, as an
   app seals and opens rows one by one; `FakeContentCipher` in tests) makes a
   store's data keys, sealed to the account and signed by it, opens them, and
   seals and opens a row's content for the row it is stored in (its table, id
-  and column), the payload holding the signature before the ciphertext. An
-  anonymous person's device makes key pairs of its own
-  (`AccountCrypto.newDeviceKeys`), which `makePassword` keeps as the account's
-  once they set a password.
+  and column), the payload holding the signature before the ciphertext.
 - **A password and TOTP** (`lib/src/password/`, ADR 0017): `PasswordRepository`
   on a `PasswordGateway` (`ServerpodPasswordGateway`), an `AccountCrypto`
   (`SodiumAccountCrypto`: halkyon_keys and SRP's client half, each step on an
@@ -125,9 +124,8 @@ Written as the apps are: Dart 3.13's primary constructors, Material from
   code or opened; and anything a step didn't expect (bytes of the wrong
   length from an instance, libsodium's or the keyring's own errors) fails it
   rather than leaving its sheet busy. `register_interop_test.dart` holds the
-  real cryptography to the register's own checks. Anonymous people get no
-  keys until something is encrypted (step 4). Every build of an app needs a C
-  compiler and make: sodium's build hook compiles libsodium.
+  real cryptography to the register's own checks. Every build of an app
+  needs a C compiler and make: sodium's build hook compiles libsodium.
 - **Signing in with SimpleLogin** (`lib/src/simplelogin/`, ADR 0020):
   `SimpleLoginRepository` on a `SimpleLoginGateway` (`ServerpodSimpleLoginGateway`,
   halkyon_base's `simpleLogin` endpoint) and halkyon_oidc_flutter's
@@ -143,18 +141,17 @@ Written as the apps are: Dart 3.13's primary constructors, Material from
   keys on the device before any proof is sent; an account opens its keys
   with its password; an account of before gets its keys, and its handle
   where it has none. With `link`, SimpleLogin joins the account of the
-  device's person instead, an anonymous person's device keys kept. The
+  device's person instead. The
   screens: the start screen's Continue with SimpleLogin, first where the
   instance offers it, and `showSimpleLoginSheet` (`SimpleLoginCubit`),
   which waits for the browser with Cancel and stops waiting on the loopback
-  once it closes. Adding it to an account (ADR 0020, D21, D23): where the
+  once it closes. Adding it to an account (ADR 0020, D21): where the
   person stands (`SimpleLoginRepository.standing`, read anew after every
   change, `SimpleLoginStandingCubit`: nothing to show, linked, or
   missing), shown by `SimpleLoginSection` (each app places it above
   `AccountSecuritySection`: a heading and Add SimpleLogin; once added a row
   among the ways in), which reads it once where a
-  `SimpleLoginStandingScope` is above it. Where the instance offers
-  SimpleLogin, GitHub is added to nobody (`GitHubOffer.linking`).
+  `SimpleLoginStandingScope` is above it.
 - **Files** (`FileSaver`, `PlatformFileSaver`): keeping a file the app made
   where the person chooses (a recovery phrase, recovery codes, an app's
   exports): on Linux and macOS the own user's alone (`writePrivately`). On