Reopen estimate: carry signature strokes on the estimate payload

Reopening an already-validated estimate showed an empty pad: the
strokes were stored server-side but never sent back. Embed them in the
estimate payload and load them straight into the pad, in the same wire
format PostIt captured — no separate fetch.

- Server: Signature inverse navs get STJ [JsonIgnore] (cycle-safe
  under System.Text.Json, matching ApplicationUser); the four estimate
  read endpoints Include(e => e.Signatures) so strokes ride the payload
- Client: EstimateDto.Signatures with EstimateSignatureDto (Strokes,
  Type=Pro/0|Client/1, CoordinateMax, CapturedAtUtc)
- PostIt: SignaturePadControl.Load(int[]) replaces the buffer and
  raises RedrawRequested; EstimateValidationPageViewModel.LoadExisting
  Signature() picks the perspective's side off the estimate and loads
  it on Attach
- Tests: server asserts GET estimate/{id} carries the strokes; PostIt
  VM tests cover load + side-by-perspective

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

src/PostIt/PostIt.Tests/EstimateValidationPageViewModelTests.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Generic;
 using System.Net.Http;
 using PostIt.Controls;
 using PostIt.ViewModels;
@@ -132,6 +133,51 @@ public class EstimateValidationPageViewModelTests
         Assert.Contains("client", cli.RoleLabel, StringComparison.OrdinalIgnoreCase);
     }
 
+    [Fact]
+    public void Load_existing_signature_renders_loaded_strokes_in_pad()
+    {
+        var strokes = new[] { 1, 5000, 5000 };
+        var front = new FrontOfficeApiClient(new RecordingApi(), "https://business.example/api/v1/");
+        var estimate = SampleEstimate();
+        estimate.Signatures = new List<EstimateSignatureDto>
+        {
+            new() { Type = 0, Strokes = strokes, CoordinateMax = 10_000 },
+        };
+        var vm = new EstimateValidationPageViewModel(estimate, EstimateListPerspective.Provider, front);
+
+        var pad = new SignaturePadControl();
+        vm.Attach(pad);
+
+        var snap = pad.Snapshot();
+        Assert.False(snap.IsEmpty);
+        Assert.Equal(strokes, snap.Strokes);
+        Assert.True(vm.HasSignature);
+    }
+
+    [Fact]
+    public void Load_existing_signature_picks_side_by_perspective()
+    {
+        var proStrokes = new[] { 1, 1000, 1000 };
+        var cliStrokes = new[] { 1, 9000, 9000 };
+        var front = new FrontOfficeApiClient(new RecordingApi(), "https://business.example/api/v1/");
+        var estimate = SampleEstimate();
+        estimate.Signatures = new List<EstimateSignatureDto>
+        {
+            new() { Type = 0, Strokes = proStrokes },
+            new() { Type = 1, Strokes = cliStrokes },
+        };
+
+        var proPad = new SignaturePadControl();
+        var proVm = new EstimateValidationPageViewModel(estimate, EstimateListPerspective.Provider, front);
+        proVm.Attach(proPad);
+        Assert.Equal(proStrokes, proPad.Snapshot().Strokes);
+
+        var cliPad = new SignaturePadControl();
+        var cliVm = new EstimateValidationPageViewModel(estimate, EstimateListPerspective.Client, front);
+        cliVm.Attach(cliPad);
+        Assert.Equal(cliStrokes, cliPad.Snapshot().Strokes);
+    }
+
     private sealed class RecordingApi : IYavscApiClient
     {
         public HttpClient Http { get; } = new();