Remove supra brand from design system

Supra (VivoKey MD3 theme) has been extracted into a standalone
Dangerous-Pi theme plugin. Remove all supra references from tokens,
web CSS, showcase apps, and documentation.

- Delete supra brand tokens and elevation.css
- Update ThemeBrand type to "dt" | "classic"
- Remove supra from CSS generation, imports, and bevels
- Update CLAUDE.md and package READMEs

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

packages/web/src/index.css

@@ -10,12 +10,8 @@
 /* Brand tokens — generated from @dangerousthings/tokens */
 @import './tokens/dt.css';
 @import './tokens/classic.css';
-@import './tokens/supra.css';
 
 /* Component styles — DT visual identity */
 @import './components/bevels.css';
 @import './components/glows.css';
 @import './components/forms-dt.css';
-
-/* Component styles — Supra/MD3 visual identity */
-@import './components/elevation.css';