Make invoice PAID stamp smaller and semi-transparent

Reduce font 80→52, border 5→3, add 35% opacity so stamp no longer
obscures line items on dense invoices.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/PowderCoating.Application/Services/PdfService.cs

@@ -175,13 +175,14 @@ public class PdfService : IPdfService
         container
             .AlignCenter()
             .AlignMiddle()
+            .Opacity(0.35f)
             .Rotate(-45f)
-            .Border(5)
+            .Border(3)
             .BorderColor(Colors.Green.Darken2)
-            .PaddingVertical(14)
+            .PaddingVertical(10)
-            .PaddingHorizontal(28)
+            .PaddingHorizontal(20)
             .Text("PAID")
-            .FontSize(80)
+            .FontSize(52)
             .Bold()
             .FontColor(Colors.Green.Darken2);
     }