Enable credential autofill for signup and login (#56)

frontend/app/test/auth_test.dart

@@ -55,6 +55,12 @@ void main() {
       await submit(tester);
       expect(find.text('Enter your email address.'), findsOneWidget);
       expect(find.text('Enter your password.'), findsOneWidget);
+      expect(
+        tester.testTextInput.log.where(
+          (call) => call.method == 'TextInput.finishAutofillContext',
+        ),
+        isEmpty,
+      );
       await tester.enterText(
         find.byKey(const ValueKey('email')),
         'test@example.com',
@@ -79,6 +85,12 @@ void main() {
       await tester.testTextInput.receiveAction(TextInputAction.done);
       await tester.pumpAndSettle();
       expect(find.byKey(const ValueKey('preview-result')), findsOneWidget);
+      expect(
+        tester.testTextInput.log,
+        contains(
+          isMethodCall('TextInput.finishAutofillContext', arguments: true),
+        ),
+      );
       expect(tester.takeException(), isNull);
     },
   );
@@ -104,12 +116,26 @@ void main() {
       );
       await submit(tester);
       expect(find.text('Passwords do not match.'), findsOneWidget);
+      expect(
+        tester.testTextInput.log,
+        isNot(
+          contains(
+            isMethodCall('TextInput.finishAutofillContext', arguments: true),
+          ),
+        ),
+      );
       await tester.enterText(
         find.byKey(const ValueKey('confirmation')),
         'example-password',
       );
       await submit(tester);
       expect(find.byKey(const ValueKey('verification-sent')), findsOneWidget);
+      expect(
+        tester.testTextInput.log,
+        contains(
+          isMethodCall('TextInput.finishAutofillContext', arguments: true),
+        ),
+      );
       await tester.ensureVisible(
         find.byKey(const ValueKey('demo-confirmation')),
       );