fix(richtext): blank lines survive into PDFs and read-only previews

Quill 2's semantic HTML saves a blank line as a truly empty <p></p> (no
<br> placeholder) — verified in the DB (invoice_sections hold <p></p>,
zero <p><br>) — which collapses to nothing outside the editor.
cleanQuillHtml (PDF path) and the new restoreQuillBlankLines (order/
invoice read-only previews) restore the placeholder at render time, so
existing stored content is covered retroactively.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/admin/pages/InvoiceDetail.tsx

@@ -58,6 +58,7 @@ import {
   formatCurrency,
   formatDate,
   numberOr,
+  restoreQuillBlankLines,
   todayLocalStr,
 } from "../utils/formatters";
 import { normalizeDateStr } from "../utils/attendanceHelpers";
@@ -1670,7 +1671,9 @@ export default function InvoiceDetail() {
                   {(s.content || "").replace(/<[^>]*>/g, "").trim() && (
                     <RichTextView
                       dangerouslySetInnerHTML={{
-                        __html: DOMPurify.sanitize(s.content || ""),
+                        __html: DOMPurify.sanitize(
+                          restoreQuillBlankLines(s.content),
+                        ),
                       }}
                     />
                   )}
@@ -1690,7 +1693,9 @@ export default function InvoiceDetail() {
           invoice.internal_notes !== "<p><br></p>" ? (
             <RichTextView
               dangerouslySetInnerHTML={{
-                __html: DOMPurify.sanitize(invoice.internal_notes),
+                __html: DOMPurify.sanitize(
+                  restoreQuillBlankLines(invoice.internal_notes),
+                ),
               }}
             />
           ) : (