fix(auth): only a session made since dismisses the owner's warning

The full reading's L1: the warning that ten wrong proofs ended a session is
there for the owner to learn of it, and any other session of the account
(another app on the same stolen phone) dismissed it. Now only a session
made after the warning dismisses it (signed in again, or the sign-in that
first shows it); an older one is refused (PasswordRefusal.signedInBefore,
its own text in English and German), and a new password clears it as
before. A member tells the register when its session asking was made.

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_base_server/test/integration/register/register_test.dart

@@ -613,7 +613,21 @@ void main() {
             expect(answer.verifiesServer(proof.serverProof), isTrue);
             expect(proof.passwordKeys.toJson(), device.passwordKeys.toJson());
             expect(proof.sessionEndedForProofsAt, warned);
-            await remote.dismissSessionEndedWarning(session, annaOnTodo);
+            // An older session of the member's dismisses nothing (the full
+            // reading's L1); one made since does.
+            await expectLater(
+              remote.dismissSessionEndedWarning(
+                session,
+                annaOnTodo,
+                sessionMadeAt: warned,
+              ),
+              refused(PasswordRefusal.signedInBefore),
+            );
+            await remote.dismissSessionEndedWarning(
+              session,
+              annaOnTodo,
+              sessionMadeAt: warned.add(const Duration(seconds: 1)),
+            );
             expect(
               (await remote.signInWaysOf(
                 session,