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/Program.cs

@@ -55,7 +55,11 @@ internal class Program
                 });
             })
             .AddYavscCors(builder.Configuration)
-            .AddControllers();
+            .AddControllersWithViews();
+
+        // Binds the "Site" section (Bills, TempDir, Avatars, …) used by
+        // the document-generation flow and BillingController.
+        services.Configure<SiteSettings>(builder.Configuration.GetSection("Site"));
 
         // accepts any access token issued by identity server
         services.AddAuthentication("Bearer")