feat(network): add opt-in allow local/private network setting

Add a setting that relaxes the SSRF guard so extensions and built-in network code can reach private/local/loopback targets, for users routing traffic through a local proxy or custom DNS. Disabled by default. Wired end-to-end: Go backend (SetAllowPrivateNetwork toggles isPrivateIP guard), Android/iOS platform bridge, Dart settings model/provider, and a toggle in Download settings.

lib/providers/settings_provider.dart

@@ -125,6 +125,12 @@ class SettingsNotifier extends Notifier<AppSettings> {
     ).catchError((Object e) {
       _log.w('Failed to sync network compatibility options to backend: $e');
     });
+
+    PlatformBridge.setAllowPrivateNetwork(state.allowLocalNetwork).catchError((
+      Object e,
+    ) {
+      _log.w('Failed to sync allow local network option to backend: $e');
+    });
   }
 
   void _syncExtensionFallbackSettingsToBackend() {
@@ -575,6 +581,12 @@ class SettingsNotifier extends Notifier<AppSettings> {
     _syncNetworkCompatibilitySettingsToBackend();
   }
 
+  void setAllowLocalNetwork(bool enabled) {
+    state = state.copyWith(allowLocalNetwork: enabled);
+    _saveSettings();
+    _syncNetworkCompatibilitySettingsToBackend();
+  }
+
   void setSongLinkRegion(String region) {
     final normalized = _normalizeSongLinkRegion(region);
     state = state.copyWith(songLinkRegion: normalized);