fix(demo-04): Update attention visualizer to use shared theming system

- Remove hardcoded purple gradient body background that prevented theme switching
- Add hero-header styling matching Eliza demo (gradient text effect)
- Convert all color references to shared CSS variables:
  - white → var(--surface-color)
  - #666 → var(--text-secondary)
  - var(--background-light) → var(--bg-secondary)
- Remove conflicting :root color variable overrides
- Demo now properly supports dark/light theme toggling

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Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: jeremymanning