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/__tests__/pdf-shared.test.ts

@@ -32,6 +32,17 @@ describe("cleanQuillHtml — inline-style whitelist", () => {
     );
   });
 
+  it("restores Quill 2's empty <p></p> blank lines to <p><br></p>", () => {
+    // Quill 2's semantic HTML drops the <br> placeholder from blank lines.
+    expect(cleanQuillHtml("<p>A</p><p></p><p>B</p>")).toBe(
+      "<p>A</p><p><br></p><p>B</p>",
+    );
+    // Attribute-carrying and whitespace-only empties too.
+    expect(cleanQuillHtml('<p class="ql-align-center"> </p>')).toBe(
+      '<p class="ql-align-center"><br></p>',
+    );
+  });
+
   it("keeps stripping scripts, event handlers and js: URLs", () => {
     const out = cleanQuillHtml(
       '<p onclick="alert(1)"><script>alert(1)</script>' +