feat: add playback gain methods to all platforms

ios/Classes/LiblinphoneFlutterPlugin.swift

@@ -244,6 +244,23 @@ public class LiblinphoneFlutterPlugin: NSObject, FlutterPlugin {
         case "getMicGain":
             result(linphoneBridge.getMicGain())
 
+        case "setPlaybackGain":
+            guard let args = call.arguments as? [String: Any],
+                  let level = args["level"] as? String else {
+                result(FlutterError(
+                    code: "INVALID_ARGUMENTS",
+                    message: "Missing required arguments",
+                    details: nil
+                ))
+                return
+            }
+            
+            linphoneBridge.setPlaybackGain(level: level)
+            result(success)
+
+        case "getPlaybackGain":
+            result(linphoneBridge.getPlaybackGain())
+
         default:
             result(FlutterMethodNotImplemented)
         }