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- Archive outdated design experiments and Discord-specific assets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

@@ -6,7 +6,7 @@ import { Avatar } from '../ui/Avatar';
 import { MemberListToggleButton } from '../layout/MemberListToggleButton';
 import { LoadingSpinner } from '../ui/LoadingSpinner';
 import { api } from '../../api/client';
-import type { Friend, FriendRequest } from '@opencord/shared';
+import type { Friend, FriendRequest } from '@backspace/shared';
 
 type Tab = 'online' | 'all' | 'pending' | 'add';
 
@@ -143,7 +143,7 @@ export function FriendsPage() {
         return (
           <div className="flex-1 p-8">
             <h2 className="text-base font-bold text-discord-text-primary uppercase mb-2">Add Friend</h2>
-            <p className="text-sm text-discord-text-muted mb-4">You can add friends with their Opencord username.</p>
+            <p className="text-sm text-discord-text-muted mb-4">You can add friends with their Backspace username.</p>
             <form onSubmit={handleAddFriend} className="relative mb-8">
               <input
                 type="text"