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/wwwroot/js/gift-certificate-bulk.js

@@ -0,0 +1,37 @@
+(function () {
+    var qtyInput = document.getElementById('Quantity');
+    var amtInput = document.getElementById('Amount');
+    var preview = document.getElementById('batchPreview');
+    var prevQty = document.getElementById('prevQty');
+    var prevAmt = document.getElementById('prevAmt');
+    var prevTotal = document.getElementById('prevTotal');
+
+    function updatePreview() {
+        var qty = parseInt(qtyInput.value, 10);
+        var amt = parseFloat(amtInput.value);
+        if (qty > 0 && amt > 0) {
+            prevQty.textContent = qty;
+            prevAmt.textContent = '$' + amt.toFixed(2);
+            prevTotal.textContent = '$' + (qty * amt).toFixed(2);
+            preview.style.display = '';
+        } else {
+            preview.style.display = 'none';
+        }
+    }
+
+    if (qtyInput && amtInput) {
+        qtyInput.addEventListener('input', updatePreview);
+        amtInput.addEventListener('input', updatePreview);
+        updatePreview();
+    }
+
+    // Disable submit button after first click to prevent double-submit during long creation
+    var form = document.querySelector('form');
+    var submitBtn = document.getElementById('submitBtn');
+    if (form && submitBtn) {
+        form.addEventListener('submit', function () {
+            submitBtn.disabled = true;
+            submitBtn.innerHTML = '<span class="spinner-border spinner-border-sm me-2"></span>Creating...';
+        });
+    }
+}());