feat: add IDataProvider interface and LocalJsonProvider with JSON file CRUD

One JSON file per machine named by GUID, stored in a configurable directory.
Supports save, load, list (as summaries), and delete with IO-error retry.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

OpenNest.Data/IDataProvider.cs

@@ -0,0 +1,9 @@
+namespace OpenNest.Data;
+
+public interface IDataProvider
+{
+    IReadOnlyList<MachineSummary> GetMachines();
+    MachineConfig? GetMachine(Guid id);
+    void SaveMachine(MachineConfig machine);
+    void DeleteMachine(Guid id);
+}