feat: add Backspace design prototype and rebrand from Opencord

- Add Backspace-design-prototype.html: finalized "Aether Drift" design
  language with warm matte surfaces and subtle frosted glass accents
- Update CLAUDE.md with DESIGN SYSTEM section and remove Discord clone references
- Rename all Opencord references to Backspace across the full codebase
- Archive outdated design experiments and Discord-specific assets
- Add science-backed accessibility fallback (prefers-reduced-transparency)

packages/web/src/components/modals/NewDmModal.tsx

@@ -5,7 +5,7 @@ import { Avatar } from '../ui/Avatar';
 import { useUIStore } from '../../stores/uiStore';
 import { useServerStore } from '../../stores/serverStore';
 import { api } from '../../api/client';
-import type { User } from '@opencord/shared';
+import type { User } from '@backspace/shared';
 
 export function NewDmModal() {
   const [query, setQuery] = useState('');