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/utils/pdf-shared.ts

@@ -141,6 +141,10 @@ export function cleanQuillHtml(html: string | null | undefined): string {
   );
   // Replace   with regular space (outside of tags)
   s = s.replace(/( )/g, " ");
+  // Quill 2's semantic HTML saves blank lines as truly empty <p></p>, which
+  // collapse to zero height in the PDF — restore the <br> placeholder so an
+  // editor blank line stays a blank line.
+  s = s.replace(/<p([^>]*)>\s*<\/p>/gi, "<p$1><br></p>");
   // Inline styles: Quill writes text/background colors as style attributes.
   // Keep ONLY color/background-color with literal color values — everything
   // else (url(), expression(), positioning…) must never reach Puppeteer.