review: gate peer-proxy toggle to FFI-supported platforms

Three review comments converged on the same root cause: the toggle was
gated to PlatformUtils.isDesktop and the platform-service shims invoked
MethodChannel methods that have no native handlers anywhere
(Android/iOS/macOS), so on any non-FFI platform the toggle would
render but the call would fail with MissingPluginException.

* vpn_setting.dart: gate to PlatformUtils.isFFISupported (Windows +
  Linux), where the FFI path actually drives the toggle.
* radiance_settings_providers.dart: skip the isPeerProxyEnabled probe
  in _refresh on non-FFI platforms so we don't log a failure on every
  settings init.
* lantern_platform_service.dart: replace the MethodChannel passthroughs
  with explicit "not supported on this platform" stubs. They exist
  only for LanternCoreService interface conformance; the UI gate
  prevents them from ever being called.

macOS / iOS / Android support requires a native handler (Swift /
Kotlin) calling into the Go core; that's a follow-up.

flutter analyze: clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Adam Fisk

lib/features/home/provider/radiance_settings_providers.dart

@@ -29,19 +29,22 @@ class RadianceSettings extends _$RadianceSettings {
     final routingF = svc.isSmartRoutingEnabled();
     final telemetryF = svc.isTelemetryEnabled();
     final splitF = PlatformUtils.isIOS ? null : svc.isSplitTunnelingEnabled();
-    final peerProxyF = svc.isPeerProxyEnabled();
+    // Peer-proxy probe runs only on platforms with native handlers
+    // (FFI-supported = Windows + Linux). On other platforms the call would
+    // fail with MissingPluginException on every settings init.
+    final peerF = PlatformUtils.isFFISupported ? svc.isPeerProxyEnabled() : null;
 
     final results = await Future.wait([
       blockAdsF,
       routingF,
       telemetryF,
       ?splitF,
-      peerProxyF,
+      ?peerF,
     ]);
     if (!ref.mounted) return;
 
     const defaults = RadianceSettingsState();
-    final peerIdx = splitF == null ? 3 : 4;
+    final peerIdx = 3 + (splitF == null ? 0 : 1);
     state = RadianceSettingsState(
       blockAds: results[0].fold((_) => defaults.blockAds, (v) => v),
       routingMode: results[1].fold(
@@ -52,7 +55,9 @@ class RadianceSettings extends _$RadianceSettings {
       splitTunneling: splitF == null
           ? defaults.splitTunneling
           : results[3].fold((_) => defaults.splitTunneling, (v) => v),
-      peerProxy: results[peerIdx].fold((_) => defaults.peerProxy, (v) => v),
+      peerProxy: peerF == null
+          ? defaults.peerProxy
+          : results[peerIdx].fold((_) => defaults.peerProxy, (v) => v),
     );
   }
 

lib/features/setting/vpn_setting.dart

@@ -120,7 +120,7 @@ class VPNSetting extends HookConsumerWidget {
             },
           ),
         ),
-        if (PlatformUtils.isDesktop) ...{
+        if (PlatformUtils.isFFISupported) ...{
           SizedBox(height: 16),
           AppCard(
             padding: EdgeInsets.zero,

lib/lantern/lantern_platform_service.dart

@@ -288,28 +288,21 @@ class LanternPlatformService implements LanternCoreService {
     }
   }
 
+  // Peer-proxy has no native MethodChannel handler outside the FFI path.
+  // The toggle is gated to PlatformUtils.isFFISupported in the UI, so these
+  // implementations exist for interface conformance only and return a clear
+  // error rather than a MissingPluginException if ever invoked.
   @override
   Future<Either<Failure, Unit>> setPeerProxyEnabled(bool enabled) async {
-    try {
-      await _methodChannel.invokeMethod('setPeerProxyEnabled', {
-        'enabled': enabled,
-      });
-      return right(unit);
-    } catch (e, st) {
-      appLogger.error('setPeerProxyEnabled failed', e, st);
-      return Left(e.toFailure());
-    }
+    return Left(Failure(
+      error: 'peer-proxy not supported on this platform',
+      localizedErrorMessage: 'peer-proxy not supported on this platform',
+    ));
   }
 
   @override
   Future<Either<Failure, bool>> isPeerProxyEnabled() async {
-    try {
-      final res = await _methodChannel.invokeMethod<bool>('isPeerProxyEnabled');
-      return right(res ?? false);
-    } catch (e, st) {
-      appLogger.error('isPeerProxyEnabled failed', e, st);
-      return Left(e.toFailure());
-    }
+    return right(false);
   }
 
   @override