fix(app): a sign-out or deletion that failed says so, its tile showing it under way

The account's tiles show a spinner on Sign out or Delete account while it
runs, start neither again meanwhile, and say once, in a snack bar, why one
failed ("Couldn't delete your account: the instance wasn't reached. Try
again."; "Couldn't sign out on this device. Try again."), in English and
German. The session gate no longer builds the person's app anew for that:
only the tiles follow it.

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/test/auth/widget/session_gate_test.dart

@@ -225,4 +225,36 @@ void main() {
     await tester.pumpAndSettle();
     expect(find.text('Signed out'), findsOneWidget);
   });
+
+  testWidgets("a sign-out under way or failed leaves the person's app as it "
+      'is, built once', (tester) async {
+    gateway = FakeAuthGateway(
+      status: const SignedIn(authUserId: 'ada', isIdentified: true),
+    )..signOutError = Exception('The keyring is locked');
+    dependencies = TestAppDependencies(gateway: gateway);
+    var builds = 0;
+    await tester.pumpWidget(
+      AppFrameScope(
+        frame: testFrame(),
+        child: AppScope(
+          dependencies: dependencies,
+          child: SessionGate(
+            look: (context) => (themeMode: ThemeMode.light, locale: null),
+            app: (person) {
+              builds++;
+              return _PersonApp(person);
+            },
+          ),
+        ),
+      ),
+    );
+    await tester.pumpAndSettle();
+    expect(builds, 1);
+
+    await tester.tap(find.text('Sign out'));
+    await tester.pumpAndSettle();
+
+    expect(find.text('App of ada'), findsOneWidget);
+    expect(builds, 1, reason: "busy and its problem are the account tiles'");
+  });
 }