Draw estimate signatures as vector TikZ instead of PNG

The estimate foot now renders the provider and client signatures as
TikZ \draw/\fill paths straight from the captured stroke data, in two
side-by-side minipages with a \useasboundingbox so the boxes align
regardless of stroke coverage. A side that hasn't signed shows a
signature rule.

- TeXHelpers.SignatureToTikz walks the PostIt wire format
  ([k, x0,y0, ...] per stroke, coords in [0, CoordinateMax], y-down)
  and emits one \draw per stroke (round caps/joins, 1.5pt), flipping y
  so the signature is upright on paper; a single-point tap becomes a
  filled dot, mirroring the Avalonia round-capped Polyline.
- FrontOfficeApiController.LoadEstimateForRenderAsync includes
  Estimate.Signatures so SignaturePro/SignatureClient resolve.
- Estimate_tex.cshtml drops the \includegraphics of a PNG (which
  failed the build when the file was absent) and adds \usepackage{tikz}.
- README: document texlive-pictures (tikz) and texlive-fonts-extra
  (bera) as runtime deps; bera.sty missing is the likely cause of the
  remaining host Emergency stop.

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

src/Yavsc.Api/Controllers/Business/FrontOfficeApiController.cs

@@ -157,12 +157,14 @@ namespace Yavsc.ApiControllers
       }
 
         // Loads the estimate linked to a query with the navigation the
-        // Estimate_tex template reads (client + performer profile + bill).
+        // Estimate_tex template reads (client + performer profile + bill
+        // + the captured signatures, drawn as tikz at the foot of the devis).
         private Task<Estimate?> LoadEstimateForRenderAsync(long queryId, CancellationToken token)
             => dbContext.Estimates
                 .Where(e => e.CommandId == queryId)
                 .OrderByDescending(e => e.Id)
                 .Include(e => e.Bill)
+                .Include(e => e.Signatures)
                 .Include(e => e.Query).ThenInclude(q => q!.Client).ThenInclude(c => c!.PostalAddress)
                 .Include(e => e.Query).ThenInclude(q => q!.PerformerProfile).ThenInclude(p => p!.OrganizationAddress)
                 .Include(e => e.Query).ThenInclude(q => q!.PerformerProfile).ThenInclude(p => p!.Performer)