style(playground): unify dark theme with orange accents and improve RL button layout
- Replace fragmented green/blue/orange accent scheme with unified hacker
orange (#ff6a00) across all CSS, JS canvas drawing, and param palettes
- Redesign designs.html: monospace font, clickable cards, grid background,
remove A/B/C labels; copy to index.html as landing page
- Add JetBrains Mono font across all views for consistent techy aesthetic
- Sharpen border-radius (pills 20px→6px, back buttons→6px) for angular feel
- Restructure RL buttons: key numbers inside buttons (larger, white),
teal-green for thumbs-up, dark red-orange for thumbs-down
- Fix RL button z-index (25→45) so they render above the bottom sheet
- Add Xavier-scaled weight randomization (spread parameter)
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