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/Yavsc.Api.Client/Dtos/EstimateDtos.cs

@@ -39,6 +39,37 @@ public sealed class EstimateDto
     public string CommandType { get; set; } = string.Empty;
     public DateTime ProviderValidationDate { get; set; }
     public DateTime ClientValidationDate { get; set; }
+
+    /// <summary>
+    /// Signatures already captured against this estimate (Pro and/or
+    /// Client), as returned with the estimate payload. Each carries the
+    /// PostIt wire-format <see cref="EstimateSignatureDto.Strokes"/>
+    /// so the pad can repaint a previously drawn signature on reopen
+    /// without a second round-trip. Null/empty when nobody has signed.
+    /// </summary>
+    public List<EstimateSignatureDto>? Signatures { get; set; }
+}
+
+/// <summary>
+/// One stored signature on an estimate, mirroring the JSON shape of
+/// the server-side <c>Yavsc.Models.Billing.Signature</c> entity. The
+/// <see cref="Strokes"/> are the same PostIt wire-format payload that
+/// was captured and are loaded back into the pad unchanged.
+/// </summary>
+public sealed class EstimateSignatureDto
+{
+    public long Id { get; set; }
+    public long EstimateId { get; set; }
+    /// <summary>
+    /// Who signed, as the integer value of the server
+    /// <c>SignatureType</c> enum: <c>0</c> = Pro (provider),
+    /// <c>1</c> = Client. Matched by value on the client so PostIt
+    /// need not reference the server enum.
+    /// </summary>
+    public int Type { get; set; }
+    public int CoordinateMax { get; set; }
+    public DateTime CapturedAtUtc { get; set; }
+    public int[]? Strokes { get; set; }
 }
 
 /// <summary>