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/utils/formatters.ts

@@ -76,3 +76,15 @@ export function czechPlural(
   if (n >= 2 && n <= 4) return few;
   return many;
 }
+
+/**
+ * Quill 2's semantic HTML saves blank lines as truly empty <p></p>, which
+ * collapse to zero height outside the editor — restore the <br> placeholder
+ * before rendering stored rich text read-only. (The PDF path does the same
+ * in cleanQuillHtml.)
+ */
+export function restoreQuillBlankLines(
+  html: string | null | undefined,
+): string {
+  return (html || "").replace(/<p([^>]*)>\s*<\/p>/gi, "<p$1><br></p>");
+}