Switch to XeLaTeX: apt-install TeX Live (xelatex/luatex/latexmk + extras); remove Tectonic install in postBuild

build(space): install fonts-raleway and fonts-lato in postBuild to satisfy fontspec without modifying template

feat(tectonic): robust detection of binary in common paths + keep python -m fallback; simplify postBuild; avoid pip tectonic

build(space): remove packages.txt; install Tectonic via postBuild using install_tectonic.sh