Pull request: feat(web): PDF bookmarks sit on the left and their sections collapse #320

The PDF viewer's outline was a flat "Contents" dropdown above the pages, and nested sections couldn't collapse.

It is now a bookmarks pane on the left of the pages, toggled from the toolbar. Entries with children expand and collapse (top level shows first), and the current section stays highlighted, or the collapsed entry that hides it. The tree logic is visiblePdfOutline in pdfLayout.ts, with a test.

Closes #301

🤖 Generated with Claude Code

Copy of Forgejo pull request #302 (merged, into main), kept so issue numbers match Forgejo.