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- Archive outdated design experiments and Discord-specific assets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

@@ -53,12 +53,12 @@ export function AppLayout() {
 
   // MutationObserver: neutralize rogue LiveKit <audio> elements that bypass our Web Audio pipeline.
   // LiveKit can re-attach hidden <audio> elements after .detach(), causing full-volume playback
-  // that ignores our volume/mute controls. Any <audio> without data-opencord is immediately killed.
+  // that ignores our volume/mute controls. Any <audio> without data-backspace is immediately killed.
   useEffect(() => {
     const observer = new MutationObserver((mutations) => {
       for (const mutation of mutations) {
         mutation.addedNodes.forEach((node) => {
-          if (node instanceof HTMLAudioElement && !node.dataset.opencord) {
+          if (node instanceof HTMLAudioElement && !node.dataset.backspace) {
             node.muted = true;
             node.volume = 0;
             node.pause();
@@ -210,7 +210,7 @@ export function AppLayout() {
             <circle className="opacity-25" cx="12" cy="12" r="10" stroke="currentColor" strokeWidth="4" />
             <path className="opacity-75" fill="currentColor" d="M4 12a8 8 0 018-8V0C5.373 0 0 5.373 0 12h4z" />
           </svg>
-          <p className="text-discord-text-muted">Loading Opencord...</p>
+          <p className="text-discord-text-muted">Loading Backspace...</p>
         </div>
       </div>
     );