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- Archive outdated design experiments and Discord-specific assets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/chat/Embed.tsx

@@ -22,7 +22,7 @@ export function Embed({ url }: EmbedProps) {
     // Simple fetch from our new API
     fetch(`/api/utils/metadata?url=${encodeURIComponent(url)}`, {
       headers: {
-        'Authorization': `Bearer ${localStorage.getItem('opencord_token')}`
+        'Authorization': `Bearer ${localStorage.getItem('backspace_token')}`
       }
     })
       .then(res => res.json())