Rewrite playground/ewm.html to interactive playground format

Convert static pre/code blocks to interactive playground-block sections
with textarea editors, run/reset buttons, and output panels. Add loading
overlay, CSS variables, playground-runtime.js script tag, and footer
matching the standard playground template (value_counts.html).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Co-authored-by: mrjf <180956+mrjf@users.noreply.github.com>

Author: Copilot