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- Archive outdated design experiments and Discord-specific assets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/layout/ActivityPanel.tsx

@@ -2,7 +2,7 @@ import React, { useEffect, useMemo } from 'react';
 import { useSocialStore } from '../../stores/socialStore';
 import { useUIStore } from '../../stores/uiStore';
 import { Avatar } from '../ui/Avatar';
-import type { Friend } from '@opencord/shared';
+import type { Friend } from '@backspace/shared';
 
 export function ActivityPanel() {
   const friends = useSocialStore((s) => s.friends);