[Bug]: HTML-to-PPTX export differs significantly from preview for Chinese decks on Windows PowerPoint

Summary

The official Presenton skill and the public POST /api/v3/export/html-to-any endpoint can generate PPTX files, but the exported PPTX differs significantly from the correct browser preview for a Chinese presentation.

The PDF exported from the exact same validated HTML is visually consistent with the preview. The PPTX is not.

Steps to reproduce

1. Create a valid 1280 x 720 presentation HTML with direct slide children under #presentation-slides-wrapper.
2. Use Tailwind and import these Google web fonts:
   • Noto Sans SC (400/500/700/900)
   • ZCOOL KuaiLe
3. Validate the HTML with the validator shipped in presenton/skills.
4. Export the same HTML separately through /api/v3/export/html-to-any:
   • format: pdf
   • format: pptx
5. Open the PPTX in Microsoft PowerPoint desktop on Windows.

Tested with a 40-slide Chinese middle-school teaching deck.

Expected behavior

The PPTX should remain visually close to the browser preview, including text presence, font selection, line breaks, element sizing, positioning, and image cropping.

If pixel-level fidelity is not supported, the API/skill should clearly report the limitation or offer a flattened image-based PPTX export mode.

Actual behavior

The browser preview renders correctly and the PDF from the same HTML matches it closely, but the PPTX has widespread fidelity problems:

• Web fonts are substituted or rendered with different metrics.
• Chinese text wraps differently and some punctuation moves to a separate line.
• Text boxes and decorative elements change size or position.
• Some slide content can disappear; in our case the cover title was missing while its background card remained.
• Several layouts and image crops differ noticeably from the preview.

This is not limited to LibreOffice, Google Slides, or macOS: it reproduces in Microsoft PowerPoint on Windows.

Additional context

Related to #747, but this is a different failure mode. #747 focuses on missing East Asian font declarations and CJK glyph fallback outside Windows. This report concerns broad HTML-to-PPTX visual fidelity, including missing content and layout changes, in Windows PowerPoint.

No private presentation link or classroom/student imagery is included in this public report. A minimized HTML reproduction can be prepared if needed.

Possible improvements

• Embed or explicitly map imported web fonts during PPTX export.
• Preserve browser-computed text metrics and line breaks more faithfully.
• Add regression tests comparing preview/PDF/PPTX renders for CJK decks.
• Offer an optional flattened 1280 x 720 image-per-slide PPTX mode for presentation fidelity.
• Document the editability-versus-fidelity tradeoff in the official skill.