feat(server): share machines, post settings, clamps and nest defaults

Schema v13 adds shared_settings and shared_setting_revisions behind
/api/settings: one revision-guarded JSON document per kind and key, with
every revision (deletions included) kept with who saved it. Recreating a
deleted document continues its revision count.

OpenNest.Data gains the client (RemoteSharedSettings), a synchronous store
that keeps the last copy read for offline use and refuses writes while the
server is unreachable (SharedSettingsStore), a machine provider over it, and
JSON parse/serialize entry points for clamp settings and nest defaults.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

OpenNest.Data/SharedMachineProvider.cs

@@ -0,0 +1,41 @@
+using System.Text.Json;
+
+namespace OpenNest.Data;
+
+/// <summary>
+/// The machine configurations shared through the nest server, in the same JSON as
+/// <see cref="LocalJsonProvider"/>'s files. Reads come from the store's last-read copy; saves
+/// and deletes go to the server and throw <see cref="IOException"/> when it cannot take them.
+/// </summary>
+public sealed class SharedMachineProvider(SharedSettingsStore store) : IDataProvider
+{
+    public IReadOnlyList<MachineSummary> GetMachines() =>
+        store.List(SharedSettingKinds.Machine)
+            .Select(Read)
+            .OfType<MachineConfig>()
+            .Select(machine => new MachineSummary(machine.Id, machine.Name))
+            .ToList();
+
+    public MachineConfig? GetMachine(Guid id) =>
+        store.Get(SharedSettingKinds.Machine, id.ToString("D")) is { } record ? Read(record) : null;
+
+    public void SaveMachine(MachineConfig machine) =>
+        store.Save(SharedSettingKinds.Machine, machine.Id.ToString("D"), Serialize(machine));
+
+    public void DeleteMachine(Guid id) => store.Delete(SharedSettingKinds.Machine, id.ToString("D"));
+
+    /// <summary>A machine's shared document, the same JSON its local file holds.</summary>
+    public static string Serialize(MachineConfig machine) => JsonSerializer.Serialize(machine, LocalJsonProvider.JsonOptions);
+
+    private static MachineConfig? Read(SharedSettingRecord record)
+    {
+        try
+        {
+            return JsonSerializer.Deserialize<MachineConfig>(record.Json, LocalJsonProvider.JsonOptions);
+        }
+        catch (JsonException)
+        {
+            return null;
+        }
+    }
+}