Rewrite playground/csv.html to use proper interactive playground format

Convert from old style.css/contenteditable structure to the standard
dark-theme template with CSS variables, loading overlay, playground-block
structure with textarea editors, Run/Reset buttons, and proper
import { ... } from 'tsb' statements in all code blocks.

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

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

Author: Copilot