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/lib/l10n/halkyon_app_en.arb

@@ -604,5 +604,9 @@
   "simpleLoginHandleInvalid": "A handle is 1 to 30 letters (a to z), digits or underscores.",
   "simpleLoginHandleReserved": "This handle is kept for the instance.",
   "simpleLoginHandleTaken": "Someone else has this handle.",
-  "simpleLoginTooManyHandleChecks": "Too many handles were tried. Try again in an hour."
+  "simpleLoginTooManyHandleChecks": "Too many handles were tried. Try again in an hour.",
+  "simpleLoginSectionTitle": "SimpleLogin",
+  "simpleLoginSignsYouIn": "Signs you in",
+  "simpleLoginAddHint": "Sign in with SimpleLogin, or with your Proton account.",
+  "simpleLoginAdded": "SimpleLogin added."
 }