Rewrite playground/json.html to use interactive playground template format

Convert from legacy style.css/contenteditable structure to the standard
dark-theme playground format with CSS variables, loading overlay,
playground-block/textarea structure, Run/Reset buttons, and proper
import statements.

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

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

Author: Copilot