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.Server/Models/Billing/Signature.cs

@@ -31,7 +31,7 @@ public class Signature
 
     public long EstimateId { get; set; }
 
-    [ForeignKey(nameof(EstimateId)), JsonIgnore]
+    [ForeignKey(nameof(EstimateId)), JsonIgnore, System.Text.Json.Serialization.JsonIgnore]
     public virtual Estimate Estimate { get; set; }
 
     /// <summary>
@@ -48,7 +48,7 @@ public class Signature
     [Required]
     public string SignerId { get; set; }
 
-    [ForeignKey(nameof(SignerId)), JsonIgnore]
+    [ForeignKey(nameof(SignerId)), JsonIgnore, System.Text.Json.Serialization.JsonIgnore]
     public virtual ApplicationUser Signer { get; set; }
 
     public SignatureType Type { get; set; }