feat(cli): add export-pdf command for vector PDF export

[Xiaofei-fei]

Summary

Add open-slide export-pdf CLI command that exports slides to high-quality vector PDFs using Playwright.

Usage

# Export a specific slide
open-slide export-pdf my-deck

# Export all slides
open-slide export-pdf --all

# Export to a custom directory
open-slide export-pdf my-deck --out-dir ~/Desktop

How it works

1. Builds the static site with open-slide build
2. Starts a local SPA-aware static server
3. For each slide page, navigates to the correct URL (?p=N)
4. Clones the slide canvas into a clean DOM tree (removing editor chrome: sidebar, toolbar, etc.)
5. Uses Chromium's print-to-PDF for true vector output (text is selectable/searchable)
6. Merges multi-page slides into a single PDF with pdf-lib

Key technical detail

Chromium's page.pdf() print compositor has a separate rendering pipeline that ignores CSS changes made via page.evaluate(), page.addStyleTag(), or even page.emulateMedia(). Only replacing the DOM tree itself (using cloneNode + document.body replacement) reliably removes UI chrome from the PDF output.

Output

• Format: PDF 1.7, vector (text selectable/searchable)
• Page size: 1920×1080px per slide
• Output directory: exports/ by default

Dependencies

Adds playwright and pdf-lib as dependencies to @open-slide/cli.

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