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- Archive outdated design experiments and Discord-specific assets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/ui/Avatar.tsx

@@ -1,5 +1,5 @@
 import React from 'react';
-import type { User } from '@opencord/shared';
+import type { User } from '@backspace/shared';
 import { useUIStore } from '../../stores/uiStore';
 
 interface AvatarProps {