fix: auto-generate OG thumbnail in screen mode and fix HF Space card

perf: optimize Docker build with lighter base, parallelized exports, and .dockerignore

feat: improve PDF export with screenshot-based embed rendering and cover page layout

fix: sync-template now self-updates, preserves thumbs and Seo.astro

fix: merge package.json dependencies during template sync

feat: backport dataviz page, embed export, and visual extraction from smol-training-playbook

Clean repository - remove missing LFS files