fix(metadata): send Accept-Language on Deezer requests so names follow the app language

Deezer localizes artist and genre names by IP geolocation when no language
is given, so users behind e.g. Arabic-region IPs got Arabic metadata on an
English device (VPN 'fixed' it). The app's display language is now pushed
to the backend and sent as Accept-Language, with an en-US default.

Fixes #480

lib/screens/main_shell.dart

@@ -75,6 +75,13 @@ class _MainShellState extends ConsumerState<MainShell>
     setPlaybackNormalizationEnabled(
       ref.read(settingsProvider).playbackNormalization,
     );
+    // Deezer & co. localize artist/genre names by IP unless told the app's
+    // language (issue #480).
+    unawaited(
+      PlatformBridge.setMetadataLanguage(
+        Localizations.localeOf(context).toLanguageTag(),
+      ),
+    );
   }
 
   @override

lib/services/platform_bridge.dart

@@ -1210,6 +1210,14 @@ class PlatformBridge {
     } catch (_) {}
   }
 
+  /// Tells the backend the app's display language so metadata providers
+  /// localize by it instead of IP geolocation. Best-effort.
+  static Future<void> setMetadataLanguage(String tag) async {
+    try {
+      await _channel.invokeMethod('setMetadataLanguage', {'tag': tag});
+    } catch (_) {}
+  }
+
   static Future<int> getGoLogCount() async {
     final result = await _channel.invokeMethod('getLogCount');
     return result as int;