feat(base)!: a handle at password sign-up (ADR 0020, D26, D28–D30)

Every new account gets a handle, and an account without one gets its
first as it sets a password. The handle is private until its person
goes public (D5).

- `passwordSignIn.signUp` takes a handle. `passwordSignIn.checkHandle`
  says whether a sign-up could take one, only while that sign-up's
  mailed code waits (`codeExpired` otherwise), counted per code request
  (30 an hour) and per network (60) (D29). Members ask the register.
- `password.set` takes the first handle of an account without one
  (D28); `SignInWays.handle` tells the app whether there is one.
- Each checks the handle before the code is used, so a refusal leaves
  the code for another try. An existing account at the address keeps
  its own handle where it has one, and the typed handle is still
  checked, so the answer says nothing about the address.
- `first_handle.dart` holds what SimpleLogin, the keys and passwords
  share: `refusalOfNewHandle`, `insertFirstProfile`, `hasHandle`.
- halkyon_app: one `HandleField` with its `HandleCheckCubit` (asked
  300 ms after typing pauses, only about the handle typed last; the
  instance's suggestion is free without asking) (D30). The password
  setup asks for the handle between the phrase check and the code, and
  a refusal at the code sends back to it. SimpleLogin's sign-up and
  keys steps use the same field and now check as it is typed.
- The module's README and ADR 0020 (D29, D30, the built state) amended.

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_localizations_en.dart

@@ -326,6 +326,11 @@ class HalkyonAppLocalizationsEn extends HalkyonAppLocalizations {
     return 'We emailed a code to $address. It\'s valid for 15 minutes.';
   }
 
+  @override
+  String passwordHandleIntro(String address) {
+    return 'A code is on its way to $address. Meanwhile, choose your handle: others find you by it. It stays private until you publish something or fill in your profile, and can change until your first publish.';
+  }
+
   @override
   String get passwordResendCode => 'Send a new code';
 
@@ -915,17 +920,20 @@ class HalkyonAppLocalizationsEn extends HalkyonAppLocalizations {
       'Link it again with your handle and your recovery phrase.';
 
   @override
-  String get simpleLoginHandleIntro =>
+  String get handleIntro =>
       'Others find you by your handle. It stays private until you publish something or fill in your profile, and can change until your first publish.';
 
   @override
-  String get simpleLoginHandle => 'Handle';
+  String get handleLabel => 'Handle';
 
   @override
-  String simpleLoginHandleFree(String handle) {
+  String handleFree(String handle) {
     return '@$handle is free';
   }
 
+  @override
+  String get handleChecking => 'Checking…';
+
   @override
   String get simpleLoginPasswordIntro =>
       'Choose a password: you type it when you sign in on another device. It never leaves this device: the instance can\'t read it, and can\'t reset it without what it protects being lost.';
@@ -1002,18 +1010,17 @@ class HalkyonAppLocalizationsEn extends HalkyonAppLocalizations {
       'Too many sign-ins began from this network. Try again in an hour.';
 
   @override
-  String get simpleLoginHandleInvalid =>
+  String get handleInvalid =>
       'A handle is 1 to 30 letters (a to z), digits or underscores.';
 
   @override
-  String get simpleLoginHandleReserved =>
-      'This handle is kept for the instance.';
+  String get handleReserved => 'This handle is kept for the instance.';
 
   @override
-  String get simpleLoginHandleTaken => 'Someone else has this handle.';
+  String get handleTaken => 'Someone else has this handle.';
 
   @override
-  String get simpleLoginTooManyHandleChecks =>
+  String get handleTooManyChecks =>
       'Too many handles were tried. Try again in an hour.';
 
   @override