pptxgenjs deck build: install location and QA loop

When building a .pptx with pptxgenjs outside the skill dir, require(pptxgenjs) fails because it is only preinstalled where the skill lives — npm install it in the project dir first. Always close the loop with the skill scripts: validate.py catches chart/XML defects PowerPoint would reject, then render via soffice->pdftoppm to JPGs and actually view them for overflow/overlap. A common runtime bug is referencing a coordinate variable from a previous slide inside a new forEach; scope every x/y as a const in the callback.