The free-Markdown slide preview now parses block-level content:
- ```lang fenced code blocks render with highlight.js colouring
(flutter_highlight); unknown languages fall back to plain monospace
instead of throwing.
- $$…$$ display math renders via flutter_math_fork (KaTeX), with a plain
fallback on parse errors.
Because the preview feeds the export rasterizer, code and math now also
appear in PDF/PPTX output. Adds flutter_highlight, flutter_math_fork and
highlight dependencies.
Tests: code block highlighted, math rendered, unknown language safe.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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