fix: add --standalone to pandoc so file2brl gets valid HTML

Without --standalone pandoc concatenates multiple HTML documents; file2brl
needs a single well-formed document to parse.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: taylorarndt

.github/workflows/build-epub.yml

@@ -41,8 +41,8 @@ jobs:
         continue-on-error: true
         run: |
           mkdir -p epub/brf
-          # Extract HTML from EPUB, then translate to BRF using liblouis (pure text→braille, zero TTS)
-          pandoc epub/git-going-with-github.epub -f epub -t html5 -o /tmp/book.html
+          # --standalone produces a single well-formed HTML doc; file2brl requires valid XML
+          pandoc epub/git-going-with-github.epub -f epub -t html --standalone -o /tmp/book.html
           file2brl /tmp/book.html epub/brf/git-going-with-github.brf || echo "BRF conversion failed"
           ls -lh epub/brf/ || echo "No BRF output produced"