feat(demo): redesign UI with dark sidebar layout and status indicators

Replace the new.css classless-framework demo with a custom dark-themed
interface inspired by broadcast tooling:

- Sidebar layout: 300px left panel with Connection and Capture cards,
  main area with video preview and resource list
- Status pill in header: Idle / Connecting… / Live with animated dot
- Share button: copies shareable endpoint URL to clipboard via
  navigator.clipboard, replacing the read-only URL input field
- Video placeholder: overlay icon + text hidden once a stream starts
- Resource cards: live dot, monospace URL, extension links with rel
  badges, Delete button — replaces the plain <details> elements
- Toast notifications: slide-up messages for stream start, delete,
  and clipboard copy results
- Self-contained CSS: removes CDN dependencies (new.css, xz.style
  Inter font), uses system font stack with CSS custom properties
- Responsive: collapses to single-column below 760px
- Preserves all existing functionality: URL param sync, channelId /
  b64json hidden params, ICE config, environment defaults

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

Author: LucasMaupin