Add plugin state persistence via PersistStateAttribute serialization

src/DesktopMagicPluginAPI/IPluginData.cs

@@ -50,4 +50,13 @@ public interface IPluginData
     /// <param name="message">The message to display.</param>
     /// <param name="title">The title of the message box.</param>
     void ShowMessage(string message, string title);
+
+    /// <summary>
+    /// Saves the current state of fields and properties marked with <see cref="PersistStateAttribute"/>
+    /// </summary>
+    /// <remarks>
+    /// State is automatically saved when the plugin stops, but this method can be called
+    /// to save state at any time, such as after important user interactions.
+    /// </remarks>
+    void SaveState();
 }

src/DesktopMagicPluginAPI/PersistStateAttribute.cs

@@ -0,0 +1,15 @@
+using System;
+
+namespace DesktopMagic.Api;
+
+/// <summary>
+/// Marks a field or property to be automatically persisted between plugin sessions.
+/// </summary>
+/// <remarks>
+/// The field or property must be JSON serializable. Complex types should have parameterless constructors.
+/// State is saved when the plugin stops and loaded when it starts.
+/// </remarks>
+[AttributeUsage(AttributeTargets.Field | AttributeTargets.Property, AllowMultiple = false, Inherited = true)]
+public sealed class PersistStateAttribute : Attribute
+{
+}