feat(app): adding SimpleLogin to an account (ADR 0020, D21, D23)

halkyon_app reads where the device's person stands with SimpleLogin
(nothing to show, linked, or missing) and shows it in SimpleLoginSection,
which each app places above the password part: Add SimpleLogin, once
added one more way in. GitHub is added to nobody where SimpleLogin is
offered, and Learn's Profile offers an anonymous person SimpleLogin
alone there (D23).

No cut-off day, reminder or banner (D24): with the developer the only
person on learn.halkyon.eu, the cut-off is the operator taking GitHub's
OAuth app out of the deploy's settings once that account has SimpleLogin.
ADR 0020 records it and drops step 5.

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

@@ -143,11 +143,18 @@ 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 (no
-  screen offers it yet). 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.
+  device's person instead, an anonymous person's device keys kept. 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
+  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`).
 - **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