PostIt: validate/refuse an estimate with a signature (pro or client)

Adds the client-side ability to open an estimate from the list, draw a
signature on a reusable SignaturePadView control, and accept or reject it
against /api/v1/front/query/accept|reject. The server resolves the actor
role (provider vs client) from the caller's identity, so the same page works
for either party.

- Yavsc.Api.Client: QueryAcceptanceDto + FrontOfficeApiClient (accept/reject)
- PostIt: extract SignaturePadView UserControl from SignaturePage; new
  EstimateValidationPage (VM + view) with Valider/Refuser/Effacer/Retour;
  wire OpenEstimate entry point from EstimateListPage; register client/page
  in DI + ViewLocator
- Tests: FrontOfficeApiClient + EstimateValidationPageViewModel unit tests;
  update EstimateListPageViewModelTests ctor

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

src/Yavsc.Api.Client/Dtos/QueryAcceptanceDto.cs

@@ -0,0 +1,44 @@
+using System;
+
+namespace Yavsc.Api.Client;
+
+/// <summary>
+/// Body of <c>POST api/v1/front/query/accept</c> and
+/// <c>.../query/reject</c>, mirroring the server-side
+/// <c>Yavsc.ViewModels.FrontOffice.QueryAcceptanceRequest</c>.
+/// <see cref="Strokes"/> is optional: when non-empty on accept it is
+/// the PostIt wire-format signature captured with the acceptance; null
+/// or empty means no signature (the only valid value on reject).
+/// </summary>
+public sealed class QueryAcceptanceRequestDto
+{
+    public int[]? Strokes { get; set; }
+
+    public int CoordinateMax { get; set; } = 10_000;
+
+    public DateTime? CapturedAtUtc { get; set; }
+}
+
+/// <summary>
+/// Response of <c>POST .../query/accept</c> / <c>.../query/reject</c>
+/// (<c>Ok(new { queryId, status, signature = ... })</c>). The
+/// <see cref="Signature"/> ref is non-null only when a signature was
+/// staged on accept.
+/// </summary>
+public sealed class QueryAcceptanceResponseDto
+{
+    public long QueryId { get; set; }
+
+    public string? Status { get; set; }
+
+    public QuerySignatureRefDto? Signature { get; set; }
+}
+
+public sealed class QuerySignatureRefDto
+{
+    public long Id { get; set; }
+
+    public long EstimateId { get; set; }
+
+    public string? Type { get; set; }
+}