Add interactive playground website for visual content generation

Single-page HTML app with 10 tabs: sparkline, gauge, kaomoji, heatmap,
table, histogram, dashboard, social post, thread splitter, and caption
builder. Dark theme, copy-to-clipboard, live char count, platform-aware
limits. Zero dependencies, works as static file or GitHub Pages.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: AdametherzLab