fix(auth): an account with a password deletes itself only with it and a code

A session alone deleted any account, with everything encrypted under its
keys (the second reading's lows; decided by the developer: the password
and a code, as a change of password takes). account.delete now takes a
DeletionProof (the password's SRP proof for a beginChange challenge, a
TOTP or recovery code, the host the device addressed); the register
checks it (requireOwnerToDelete), a member passing it on bound to its own
host. Accounts without a password, anonymous ones among them, delete with
their session as before. In the app, deleting an account with a password
asks for it and a code in a sheet of its own; a wrong one deletes
nothing.

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_de.dart

@@ -788,4 +788,8 @@ class HalkyonAppLocalizationsDe extends HalkyonAppLocalizations {
   String passwordCodesWait(String date) {
     return 'Zu viele falsche Codes hintereinander: Bis $date wird kein Code angenommen. Waren es nicht deine, kennt jemand dein Passwort: Ändere es dann.';
   }
+
+  @override
+  String get passwordDeleteIntro =>
+      'Dein Konto wird mit allem darin gelöscht, in jedem Dienst dieser Instanz. Gib zur Bestätigung dein Passwort und einen Code deiner Authenticator-App ein.';
 }