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- Archive outdated design experiments and Discord-specific assets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

@@ -49,7 +49,7 @@ function AudioTrackElement({
   // The <audio> element is always muted — it serves only as a Chrome
   // keep-alive so Chrome continues processing the WebRTC track.
   // Real audio output goes through the Web Audio pipeline.
-  return <audio ref={audioRef} autoPlay playsInline data-opencord="keepalive" />;
+  return <audio ref={audioRef} autoPlay playsInline data-backspace="keepalive" />;
 }
 
 /**