Estimate validation polish + PDF/TeX generation in Yavsc.Api

- List removal: GetOngoingEstimatesAsProvider filters on
  ProviderValidationDate so a provider's signed estimate leaves their
  list; PostIt refreshes the list on navigation pop-back.
- Title renames: "Devis en attente de signature fournisseur/client"
  on both the provider and client sides (HomePage, VM, tests).
- Move the estimate/bill TeX+PDF generation flow from Yavsc.Org into
  Yavsc.Api (TeXHelpers, PdfGenerationViewModel, BillViewComponent,
  Bill/Estimate templates, _ViewImports). Program.cs wires
  AddControllersWithViews + SiteSettings; FrontOfficeApiController
  injects IRazorViewEngine (cast to IViewEngine) with optional deps
  and exposes estimate.tex/estimate.pdf endpoints keyed by query id.
- PDF generation drives through the Estimate_pdf view (side-effect
  render) and compiles via the system lualatex, fed TeX on stdin (no
  intermediate .tex), PDF to the bills dir, aux artifacts cleaned so
  only the .pdf persists. Resolve the bills dir to an absolute path so
  lualatex's -output-directory and working directory agree regardless
  of the host cwd (fixes "I can't write on file estimate-1.log").
- PostIt: DownloadAsync on the client + GetEstimateTex/PdfAsync;
  Télécharger PDF/TeX buttons on the validation and command-detail
  pages via a StorageProvider save helper.

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

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

@@ -41,32 +41,34 @@ public sealed class EstimateDto
     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.
+    /// The provider's signature on this estimate, or null when the
+    /// provider has not signed. 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.
     /// </summary>
-    public List<EstimateSignatureDto>? Signatures { get; set; }
+    public EstimateSignatureDto? SignaturePro { get; set; }
+
+    /// <summary>
+    /// The client's signature on this estimate, or null when the
+    /// client has not signed.
+    /// </summary>
+    public EstimateSignatureDto? SignatureClient { 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.
+/// was captured and are loaded back into the pad unchanged. The side
+/// (Pro/Client) is implicit in which property of
+/// <see cref="EstimateDto"/> carries this DTO, so no <c>Type</c>
+/// field is needed.
 /// </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; }