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- Archive outdated design experiments and Discord-specific assets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

@@ -4,7 +4,7 @@ import { isDmChannel, useServerStore } from '../../stores/serverStore';
 import { wsSend } from '../../hooks/useWebSocket';
 import { api } from '../../api/client';
 import { MentionPopover } from './MentionPopover';
-import type { MemberWithUser } from '@opencord/shared';
+import type { MemberWithUser } from '@backspace/shared';
 
 interface MessageInputProps {
   channelId: string;