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.Web/Views/Shared/_SectionHeader.cshtml

@@ -0,0 +1,13 @@
+@* Usage: @await Html.PartialAsync("_SectionHeader", (Kicker: "FLOOR", Title: "Active jobs", Meta: (string?)null))
+   Replaces: <div class="card-header fw-bold">…</div> *@
+@model (string Kicker, string Title, string? Meta)
+<div class="pcl-section-header d-flex align-items-baseline justify-content-between gap-3">
+    <div>
+        <div class="pcl-metric-kicker">@Model.Kicker</div>
+        <h2 class="pcl-section-title mb-0">@Model.Title</h2>
+    </div>
+    @if (!string.IsNullOrEmpty(Model.Meta))
+    {
+        <span class="mono pcl-section-meta text-nowrap">@Model.Meta</span>
+    }
+</div>