Hide settings rows whose key the aircraft cannot read

The Mavic Pro remote reports GIMBAL_CONTROL_SPEED_COEFFICIENT as supported but
fails every read, so "Gimbal dial speed" sat on "—" forever. The key bridge
now reports a failed seeding read, and the row hides like an unsupported one
until a value does arrive.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Fly App/test/fly/settings_test.dart

@@ -146,6 +146,17 @@ void main() {
       expect(m.keys.value(SettingsKeys.novice), isNull);
     });
 
+    test('a supported key that cannot be read hides its row until a value arrives', () {
+      final FlightModel m = connectedModel(phantom4);
+      final TelemetryReducer r = TelemetryReducer(m);
+      m.keys.setSupported(SettingsKeys.gimbalDialSpeed, true);
+      r.apply(<String, dynamic>{'type': 'key', 'id': SettingsKeys.gimbalDialSpeed.id, 'unreadable': true});
+      expect(m.keys.supported(SettingsKeys.gimbalDialSpeed), isFalse);
+      r.apply(<String, dynamic>{'type': 'key', 'id': SettingsKeys.gimbalDialSpeed.id, 'value': 40});
+      expect(m.keys.supported(SettingsKeys.gimbalDialSpeed), isNot(isFalse));
+      expect(m.keys.value(SettingsKeys.gimbalDialSpeed), 40);
+    });
+
     test('subscriptions listen only to supported keys and cancel on dispose', () async {
       final FakeDjiService dji = FakeDjiService()..unsupportedKeys.add('ESC_BEEPING_ENABLED');
       final KeyState s = KeyState();