feat(cowriter): export markdown drafts as PDF

[KacperKazan]

Summary

• Add a reusable client-side PDF export helper backed by the existing html2canvas + jspdf dependencies.
• Add an Export PDF toolbar action to Co-Writer next to the existing Markdown export.
• Render exports with a light, e-ink-friendly A4 layout and preserve the rendered Markdown preview, including KaTeX/math and diagram output when present in the preview.

Notes

• This initial implementation intentionally avoids a server-side Pandoc dependency, so it works in the existing web app without installing external binaries.
• The generated PDF is rendered from the preview DOM, which favors visual fidelity for formulas/diagrams over selectable text.

Validation

• npm run lint -- "app/(workspace)/co-writer/[docId]/page.tsx" "lib/pdf-export.ts" (passes with existing repo warnings)
• npx tsc --noEmit --pretty false
• git diff --check
• Browser smoke test: created a temporary Co-Writer draft with Markdown, math, and Mermaid content; confirmed Export PDF button renders and completes with PDF exported and no console errors.