Restore all zeroed views + add bulk gift certificate creation

The HTML entity sweep script had a bug where it wrote empty files for any
view that contained no target Unicode characters, zeroing out 215 view files.
All views restored from the pre-sweep commit (cefdf3e).

Bulk gift certificate feature:
- BulkCreateGiftCertificateDto with Quantity (1-500), Amount, Reason, Expiry, Notes
- GenerateBulkGiftCertificatePdfAsync on IPdfService / PdfService: one Letter page
  per cert, reusing the same purple/gold branded ComposeGiftCertificateContent helper
- GiftCertificatesController: BulkCreate GET/POST, BulkResult GET, BulkDownloadPdf POST
- Views: BulkCreate.cshtml (form with live total preview), BulkResult.cshtml (table +
  Download All PDF button that POSTs cert IDs to avoid URL length limits)
- gift-certificate-bulk.js: live preview + spinner/disable on submit
- Index.cshtml: Bulk Create button added alongside New Certificate

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

src/PowderCoating.Application/Interfaces/IPdfService.cs

@@ -51,4 +51,10 @@ public interface IPdfService
         byte[]? companyLogo,
         string? companyLogoContentType,
         CompanyInfoDto companyInfo);
+
+    Task<byte[]> GenerateBulkGiftCertificatePdfAsync(
+        IList<GiftCertificateDto> certs,
+        byte[]? companyLogo,
+        string? companyLogoContentType,
+        CompanyInfoDto companyInfo);
 }